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
      
