TST: remove some test warnings in parser tests
TST: move highmemory test to proper location in c_parser_only

xref #16798

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17057?src=pr&el=h1) Report
> Merging [#17057](https://codecov.io/gh/pandas-dev/pandas/pull/17057?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/8d7d3fb545b4273cf9d1a61bf7ea3bfdde8a1199?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/17057/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&height=150&width=650)](https://codecov.io/gh/pandas-dev/pandas/pull/17057?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17057      +/-   ##
==========================================
- Coverage   91.02%   91.01%   -0.02%     
==========================================
  Files         161      161              
  Lines       49351    49351              
==========================================
- Hits        44924    44915       -9     
- Misses       4427     4436       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.78% <100%> (ø)` | :arrow_up: |
| #single | `40.26% <100%> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17057?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/conftest.py](https://codecov.io/gh/pandas-dev/pandas/pull/17057?src=pr&el=tree#diff-cGFuZGFzL2NvbmZ0ZXN0LnB5) | `96.77% <100%> (ø)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17057?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17057?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.75% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17057?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/17057?src=pr&el=footer). Last update [8d7d3fb...b9c5e36](https://codecov.io/gh/pandas-dev/pandas/pull/17057?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @jreback @gfyoung Ah, OK, now I see what file you were talking about. I missed the fact that `c_parser_only.py` was a thing...

Much better now. Thanks for cleaning up my mess!