Added tests for _get_dtype
 - [ ] closes #xxxx
 - [ x] tests added / passed
 - [ ] passes ``git diff upstream/master --name-only -- '*.py' | flake8 --diff`` (On Windows, ``git diff upstream/master -u -- "*.py" | flake8 --diff`` might work as an alternative.)
 - [ ] whatsnew entry

See #16817 for a discussion on this.

Added tests for `` pd.core.dtypes.common._get_dtype``. Some things to note:
* Tests for Categoricals fail, I've marked them xfail. This is because I propose allowing categoricals to pass in a later commit.
* ``Interval``s don't have a dtype, so they'd fail with an AttributeError. I therefore haven't added tests for Intervals, but it could be considered if it's a bug/inconsistency, that ``Interval``s don't have a dtype.

This is my first serious pull request, and I may have made errors. If anything is wrong, against the style guide,etc. please let me know.

I will make a pull request of tests for ``_get_dtype_type``, when this one has been accepted.
