CLN: Work on converting assert_'s to specialized forms, in pandas/tests

Work on #6175, in pandas/tests. Tests still pass locally. A work-in-progress, but--with #6261--removes many/most cases of assert_(x == y) in this directory

Replacements:
* assert_(x == y) ==> assertEqual(x, y)
* assert_(x == True) ==> assertTrue(x)
* assert_(x == False) ==> assertFalse(x)