Use assertion library for Pluto tests

[well-in-that-case]

The assertion library is very helpful in some instances (table comparison), but it'll always be far more useful when an error case actually occurs due to significantly improved error messages.

This should be done slowly — possibly over dozens of commits — with great attention-to-detail for each test case we convert. It's imperative we ensure the assertion library does not contain any logic errors which corrupt our test integrity. There is no known fault in the library and Pluto's test cases for the library pass as expected, but it's not a time-tested library yet.