ENH: refactor test data to run through the QIIME 2 Framework

[ChrisKeefe]

Improvement Description
Many of the tests in this package can be refactored to run through the framework. Unit tests run in isolation are still important here in some cases and will be retained, but most of these tests are less useful or reliable if they fail to take framework machinery into account.

Current Behavior
Most tests test code in isolation from the framework.

Proposed Behavior
Most tests check behavior when the plugin is integrated with the framework.

References
Raised here