compiletest: warn/error on redundant `check-fail` directives

[jieyouxu]

check-fail here is redundant

Originally posted by @compiler-errors in #130718 (comment)

Some test suites have a default test behavior, like //@ check-fail, in which case specifying that explicitly in the test is redundant and useless noise. When compiletest directive handling is worked, we should warn or error on redundant directives like these and also explain why it's redundant, e.g. "ui test mode is check-fail by default".

Remark: this check should not be added before reworking how compiletest directives are handled as it's not just one test suite or directive.

[jieyouxu]

This is E-easy but just randomly adding checks here an there isn't a maintainable solution.

[Shunpoco]

@jieyouxu
If it is not blocked by other issues, can I take this ticket?

[jieyouxu]

@Shunpoco Hi, I would prefer to not spot-fix this, and instead rework the logic such that it falls naturally out of test suite presets vs incompatible flag checking.

[Shunpoco]

rework the logic such that it falls naturally out of test suite presets vs incompatible flag checking.

I thought that I can modify TestProps's function (perhaps update_fail_mode) to warn/error on such a redundant directive, but do you mean we will reconstruct functions to achieve your thought?

[jieyouxu]

I moreso meant that it's not just //@ check-fail, that compiletest should have a centralized mechanism for checking for incompatibilities between directive <-> test suite presets <-> compile-flags. If such infra is implemented, then this particular instance should just be a matter of registering as a conflict.