add docs about reflective test authoring

[pq]

Follow-up from: dart-archive/linter#3567 (comment).

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).

[srawlins]

Now that all tests are reflective, do we still need docs encouraging their use? Should we just repurpose this ticket into a ticket to document how to write reflective tests?

[bwilkerson]

Or point people to an enhanced version of https://github.com/dart-lang/sdk/blob/main/pkg/analyzer/doc/implementation/tests.md?