TST: Parametrize dtypes tests - test_common.py and test_concat.py
Parametrized some straightforward dtypes related tests.

Relatively minor changes to `test_common.py`:
  - mostly just extracted `for` loops
  - split one test into two separate tests (see code comment)

Made larger changes to `test_concat.py`:
  - removed the test class in favor of plain functions, since there was only one class in the file
  - combined a few different tests into a single test, since they were all using test procedure
      - removed the `check_concat` validation function since it's only called once now
      - further parametrized over a `for` loop previously in the  `check_concat` validation function
  - removed the `import pandas as pd` in favor of directly importing the necessary classes
      

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20340?src=pr&el=h1) Report
> Merging [#20340](https://codecov.io/gh/pandas-dev/pandas/pull/20340?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/3783ccc694ef93c0cf159ca1ad8e69a67d828bf0?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/20340/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/20340?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #20340      +/-   ##
==========================================
- Coverage   91.76%   91.73%   -0.03%     
==========================================
  Files         150      150              
  Lines       49151    49151              
==========================================
- Hits        45102    45090      -12     
- Misses       4049     4061      +12
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.12% <ø> (-0.03%)` | :arrow_down: |
| #single | `41.9% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/20340?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/20340/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `65.07% <0%> (-1.74%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20340?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/20340?src=pr&el=footer). Last update [3783ccc...8933d34](https://codecov.io/gh/pandas-dev/pandas/pull/20340?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 made the requested changes thanks @jschendel 