extend/ENV/std: ignore fails_with during brew test.

[carlocab]

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The compiler used to a build a formula is typically not needed during
the test.

This will allow us to get rid of some :test dependencies, which were
added to prevent brew from throwing a CompilerSelectionError because
the formula declares fails_with the default compiler.

This also helps us get more accurate results from brew linkage in
cases of unintended linkage with the compiler used to build.

Fixes #11795.

[BrewTestBot]

Review period will end on 2022-09-22 at 14:56:59 UTC.

[carlocab]

The compiler used to a build a formula is typically not needed during the test.

If it is still needed, the formula can declare the appropriate :test dependency and the compiler selection logic will still use the desired one.

[Bo98]

I think this makes sense.

[BrewTestBot]

Review period ended.