timestamp/timedelta test cleanup
More informative names in timedeltas tests

remove/merge redundant tests in timestamps.  Separate some big test classes thematically.

Use pytest.parametrize in a few places
Hello @jbrockmendel! Thanks for updating the PR.





Cheers ! There are no PEP8 issues in this Pull Request. :beers: 

##### Comment last updated on December 04, 2017 at 01:22 Hours UTC # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18619?src=pr&el=h1) Report
> Merging [#18619](https://codecov.io/gh/pandas-dev/pandas/pull/18619?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/6e56195fcb021e7c53301722804087ef2a83bc39?src=pr&el=desc) will **decrease** coverage by `0.02%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18619/graphs/tree.svg?height=150&src=pr&token=eZ4WkYLtcO&width=650)](https://codecov.io/gh/pandas-dev/pandas/pull/18619?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18619      +/-   ##
==========================================
- Coverage   91.46%   91.44%   -0.03%     
==========================================
  Files         157      157              
  Lines       51449    51449              
==========================================
- Hits        47060    47049      -11     
- Misses       4389     4400      +11
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.31% <ø> (-0.01%)` | :arrow_down: |
| #single | `40.6% <ø> (-0.11%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18619?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/18619/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/18619/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `81.99% <0%> (-0.2%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/18619/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.81% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18619?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/18619?src=pr&el=footer). Last update [6e56195...9538c07](https://codecov.io/gh/pandas-dev/pandas/pull/18619?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/18619?src=pr&el=h1) Report
> Merging [#18619](https://codecov.io/gh/pandas-dev/pandas/pull/18619?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/6e56195fcb021e7c53301722804087ef2a83bc39?src=pr&el=desc) will **decrease** coverage by `0.02%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18619/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/18619?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18619      +/-   ##
==========================================
- Coverage   91.46%   91.44%   -0.03%     
==========================================
  Files         157      157              
  Lines       51449    51449              
==========================================
- Hits        47060    47049      -11     
- Misses       4389     4400      +11
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.31% <ø> (-0.01%)` | :arrow_down: |
| #single | `40.6% <ø> (-0.11%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18619?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/18619/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/18619/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `81.99% <0%> (-0.2%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/18619/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.81% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18619?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/18619?src=pr&el=footer). Last update [6e56195...9538c07](https://codecov.io/gh/pandas-dev/pandas/pull/18619?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks!