Auto merge of #123577 - Urgau:prep-work-for-compiletest-check-cfg, r=oli-obk

Do some preparation work for compiletest check-cfg

This PR does several preparation work for having always-on check-cfg in compiletest.

In particular, this PR does two main things:
 - It unifies all the *always-false* cfgs under the `FALSE` cfg (as it seems to be the convention under `tests/ui`)
 - It also removes some useless conditions

This is done ahead of the introduction of the always-on check-cfg in compiletest to reduce the amount of changes in that follow-up work. I also think that this is useful even without that follow-up work.

Author: bors