TST: Check lossiness of floats with parse_dates
The examples don't fail anymore with `read_csv`, so let's add them as tests.

Closes #2697.

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18136?src=pr&el=h1) Report
> Merging [#18136](https://codecov.io/gh/pandas-dev/pandas/pull/18136?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/1181622fbfc9208a906ae9e70716b83fc7b3ae5a?src=pr&el=desc) will **decrease** coverage by `0.03%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18136/graphs/tree.svg?src=pr&token=eZ4WkYLtcO&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/18136?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18136      +/-   ##
==========================================
- Coverage   91.28%   91.24%   -0.04%     
==========================================
  Files         163      163              
  Lines       50122    50122              
==========================================
- Hits        45752    45732      -20     
- Misses       4370     4390      +20
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.05% <ø> (-0.03%)` | :arrow_down: |
| #single | `40.32% <ø> (-0.06%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18136?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/18136?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/18136?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `63.38% <0%> (-1.82%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/18136?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.75% <0%> (-0.1%)` | :arrow_down: |
| [pandas/io/parsers.py](https://codecov.io/gh/pandas-dev/pandas/pull/18136?src=pr&el=tree#diff-cGFuZGFzL2lvL3BhcnNlcnMucHk=) | `95.57% <0%> (+0.06%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18136?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18136?src=pr&el=footer). Last update [1181622...ff98f38](https://codecov.io/gh/pandas-dev/pandas/pull/18136?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18136?src=pr&el=h1) Report
> Merging [#18136](https://codecov.io/gh/pandas-dev/pandas/pull/18136?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/1181622fbfc9208a906ae9e70716b83fc7b3ae5a?src=pr&el=desc) will **decrease** coverage by `0.03%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18136/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/18136?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18136      +/-   ##
==========================================
- Coverage   91.28%   91.24%   -0.04%     
==========================================
  Files         163      163              
  Lines       50122    50122              
==========================================
- Hits        45752    45732      -20     
- Misses       4370     4390      +20
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.05% <ø> (-0.03%)` | :arrow_down: |
| #single | `40.32% <ø> (-0.06%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18136?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/18136?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/18136?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `63.38% <0%> (-1.82%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/18136?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.75% <0%> (-0.1%)` | :arrow_down: |
| [pandas/io/parsers.py](https://codecov.io/gh/pandas-dev/pandas/pull/18136?src=pr&el=tree#diff-cGFuZGFzL2lvL3BhcnNlcnMucHk=) | `95.57% <0%> (+0.06%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18136?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18136?src=pr&el=footer). Last update [1181622...ff98f38](https://codecov.io/gh/pandas-dev/pandas/pull/18136?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @gfyoung worth adding the ``parse_dates=True`` as additional xfails here? (or maybe just open a *new* issue with that case)? maybe even show a warning that things are losing precision? @jreback : I'm not sure I follow you here.  The tests are parametrized so that they pass regardless of whether `parse_dates=True` or `False`!  😄  right but the original issues shows a loss of precision when parse_dates=True
is that fixed (original issue gives a symptom that doesn’t have an easy fix)or is it just the way it is?

 Yes, the issue is fixed on `master` (somehow).  That's why my test checks the output for `parse_dates=True` (with no `xfail`). thanks @gfyoung if you figure out where then add a reference here (not a big deal though)