Require compile-fail tests for new lang features

Its non trivial to test lang feature gates, and people
forget to add such tests. So we extend the features lint
of the tidy tool to ensure that all new lang features
contain a new compile-fail test.

Of course, one could drop this requirement and just
grep all tests in run-pass for #![feature(abc)] and
then run this test again, removing the mention,
requiring that it fails.

But this only tests for the existence of a compilation
failure. Manual tests ensure that also the correct lines
spawn the error, and also test the actual error message.

For library features, it makes no sense to require such
a test, as here code is used that is generic for all
library features.

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