Rollup merge of #143870 - jieyouxu:compiletest-maintenance-6, r=Kobzol

[COMPILETEST-UNTANGLE 6/N] Use `TestSuite` enum instead of stringly-typed test suites

This is part of a patch series to untangle `compiletest` to hopefully nudge it towards being more maintainable.

This PR should contain no functional changes.

Author: samueltardieu