TST: Fix test assertions
 - [ ] closes #xxxx
 - [ ] tests added / passed
 - [x] passes ``git diff upstream/master --name-only -- '*.py' | flake8 --diff``
 - [ ] whatsnew entry

This fixes some test assertions that were only checking the truthiness of a value when they should have been comparing two values. Initially, through typos, tests were written using assertTrue when they should have been using assertEqual. The second argument to assertTrue was being treated as a message instead of as the expected value. More recently, these assertions with typos were transformed to use `assert`, with the same problems persisting.

I have fixed each of the problems I found on a case-by-case basis. Some lines required more complicated fixes, like adding instanceof() or fixing an expected value. I believe I have found all such instances of this error, because I reviewed all the locations where assertTrue was called with two arguments.