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.
0e5f520788