TST/REF: io/parser/(test_dtypes.py, test_usecols.py)
- [ ] closes #xxxx
- [ ] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

#38370 adds a pyarrow engine to the csv reader. Only a fraction of the io/parser tests pass when pyarrow is used and the rest has to be xfailed/skipped, resulting in a large diff on the PR.

xref https://github.com/pandas-dev/pandas/pull/38370#discussion_r539579072 suggests reorganizing the tests into classes so groups of tests can be xfailed with a single mark.

I'm grouping the tests logically (not based on whether or not they pass with pyarrow) but merging this this _will_ reduce the diff in #38370 substantively.

Likely I will submit a follow-on with some further reorg. I'm happy to push that to this PR if that's preferred, though.

Verifying that total number of tests is unchanged:
```
(pandas-dev) andrewwieteska@Andrews-MacBook-Pro pandas % pytest  pandas/tests/io/parser/test_dtypes.py  pandas/tests/io/parser/test_usecols.py       
========================================================================= test session starts =========================================================================
platform darwin -- Python 3.8.6, pytest-6.1.2, py-1.9.0, pluggy-0.13.1
rootdir: /Users/andrewwieteska/repos/pandas, configfile: setup.cfg
plugins: forked-1.2.0, xdist-2.1.0, cov-2.10.1, asyncio-0.14.0, hypothesis-5.41.2, instafail-0.4.1
collected 372 items                                                                                                                                                   

pandas/tests/io/parser/test_dtypes.py ......................................................................................................................... [ 32%]
............................................................................................                                                                    [ 57%]
pandas/tests/io/parser/test_usecols.py ........................................................................................................................ [ 89%]
.......................................                                                                                                                         [100%]

======================================================================== 372 passed in 32.49s =========================================================================
```
versus on master:
```
(pandas-dev) andrewwieteska@Andrews-MacBook-Pro pandas % pytest  pandas/tests/io/parser/test_dtypes.py  pandas/tests/io/parser/test_usecols.py
========================================================================= test session starts =========================================================================
platform darwin -- Python 3.8.6, pytest-6.1.2, py-1.9.0, pluggy-0.13.1
rootdir: /Users/andrewwieteska/repos/pandas, configfile: setup.cfg
plugins: forked-1.2.0, xdist-2.1.0, cov-2.10.1, asyncio-0.14.0, hypothesis-5.41.2, instafail-0.4.1
collected 372 items                                                                                                                                                   

pandas/tests/io/parser/test_dtypes.py ......................................................................................................................... [ 32%]
............................................................................................                                                                    [ 57%]
pandas/tests/io/parser/test_usecols.py ........................................................................................................................ [ 89%]
.......................................                                                                                                                         [100%]

======================================================================== 372 passed in 32.90s =========================================================================
```