compiletest accepts header directives that are not supported by a testsuite

For example, you can add `normalize-stdout` to a `rustdoc` test, and this will be accepted.

This is unexpected because compiletest will catch nonsense/typod directives, such as `normalize-stdoutttttt`.

Ideally, if you use an incompatible directive, you will get a warning/error that tells you that it is incompatible, along with which test suites it is compatible with.