Remove test reader

[davidmorgan]

For #3811, review and merge #3881 first.

Fold internal-only testing capabilities into build_test, but keep it hidden from the public API.

[github-actions]

PR Health

Changelog Entry ❗

Package	Changed Files
package:build_resolvers	build_resolvers/lib/src/analysis_driver_model.dart

Changes to files need to be accounted for in their respective changelogs.

This check can be disabled by tagging the PR with skip-changelog-check.