TST: consolidate remaining tests under pandas.tests
 built on #15368 
move all remaining tests so that ALL tests are now under pandas/tests
looks bigger than it is :> I know there has been moving tests a lot lately, but personally, for specific self-contained submodules, I prefer the tests to be within that submodule (eg io tests in the io module).  @jorisvandenbossche you should try it. It makes it MUCH easier to do things, no more thinking about the paths, everything is *always* ``pytest pandas/tests/....``