TST: ujson tests are not being run (#16499)
add Test prefix on classes to enable PyTest picking them up.
add underscore in test method where applicable.  E.G. testBool() to test_Bool().

 - [X] closes #16499
 - [X] tests added / passed
 - [X] passes ``git diff upstream/master --name-only -- '*.py' | flake8 --diff``
 - [ ] whatsnew entry