compiletest: don't silently accept unsupported directives in test suites

[jieyouxu]

Yes, there are special handling here and there for specific directives. But the handling is limited and is only for specific directives in specific test suites.

We should error if a test writer specifies a directive invalid for a given test suite.

This may require reworking how directives are handled, i.e. have test suites "register" what directives they support.

[jieyouxu]

This is not maintainable by adding random checks everywhere in the directive handling, it's probably better as a validation pass as part of a rework of directive handling.