Added test for _get_dtype_type.
 - [ ] closes #xxxx
 - [ x] tests added / passed
 - [ x] 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

Tests for ``core.dtype.common._get_dtype_type``.