Add human-readable descriptions for all test fixtures

[wjhsf]

Currently, the labels for test fixtures are derived from the file path. This suboptimal, because descriptions should be long, while file paths should be short. The descriptions lack nuance, making it harder to understand what they do. The filenames often take up more space than they're given by the IDE, making it harder to navigate between files.

To fix this, we should add descriptions to test configurations, using the option added in #4595.