compiletest: 'known directive' check does not account for suites

[clubby789]

#134738

A few UI tests uses 'forbid-output' despite it (currently) only working for incremental tests. This is because the known directive check doesn't account for the supported suites; this makes me suspect there might be a few more noop directives in our test suite

[jieyouxu]

It's less noop directive in general, it's moreso that most(?) directives are only supported for a subset of test suites. The known directive check is a very crude and stopgap way to block new unknown (like, at all) directives from being added, but does not try to fix the preexisting problem of a directive being unsupported in certain suites.

[jieyouxu]

Part of #131425.

Anyway, I think we should fix this by having directive registration explicitly declare what test suites they do support.

[clubby789]

Closing in favour of #128058