CLN: Convert last of assert_(x == y) to assertEqual
Work on #6175. Fixes last of vanilla assert_(x == y) conversions in pandas/tests and pandas/*/tests. Tests still pass locally.

The following returns seven results (when run from pandas), but none are in the assert_(x == y) category.

grep -nH --recursive -e 'assert_(.*==' . | grep -v 'all()' | grep -v 'any()'

I will handle the easy '!=' cases and then move to refactoring some of the other ideas discussed in #6175.
 In your next PR can you make an entry in release notes (improvements section), referencing these previous PR's to say that the testing statement were updated (just so their is a record of what changed)
 @jreback, thank you, I did this in the PR I submitted a moment ago.
