add docs about reflective test authoring

[pq]

Follow-up from: https://github.com/dart-lang/linter/pull/3567#discussion_r934847261.

As a general rule we want to encourage reflective tests over integration tests. It would be great to have some docs here.

[bwilkerson]

Consider adding the docs to test_reflective_loader and referencing them from here (and from analyzer and analysis_server and any other package that uses that support).