Improve tests with inherently named tests

Refactor nearly all tests by using `map[string]TestCase` for the list of
test cases. This finishes some ongoing refactoring from recent commits.

This change has several benefits, including compiler enforced unique
test names, required names (can't forget to add it to the struct),
simpler `TestCase` types. Additionally, this change makes it more likely
to end up with more descriptive test names, which is already observed in
a few cases here.

Signed-off-by: Eric Cornelissen <ericornelissen@gmail.com>