Refactor decorator/mixin tests for future maintainability

Originally, tests were more explicitly separated, to make
sure I got all desired cases. Am now pretty confident with such.

However, the redundancy and extra code from explicit defiition
was making it overwhelming to come back to the code when
verifying/fixing additional logic.

All tests should effectively function the same and test the same
content, but syntax is reworked to be much more condensed, and
easier to modify or add new tests.