Document which test cases fall into categories of "example" and "feature test"

Anne suggests that the documentation for each test case should give users an indication at the start of the documentation of whether it falls into one or both of the following categories:
- Tests that provide good starting examples for interesting science.
- Tests that are designed to ensure a particular feature in MESA continues to function properly.

This could help users find good starting examples and avoid some of the test cases that are more narrowly tailored to test specific capabilities with a setup that isn't intended to reflect a particular astrophysical reality.

Additionally, for tests that are designed around testing specific features, perhaps the documentation could note these features at the beginning, along with the relevant inlist options that enable these features in the test.

Review the documentation for the MESA test cases and determine where each case is described; the issue does not name specific files or an entry point. Define the category labels, then document each test's example or feature-test status, relevant features, and enabling inlist options, with coverage of the full test suite as the completion criterion.

Written by the indexing model from the issue text.