UI tests that pass `--test` to rustc fail on panic=abort targets

[japaric]

Namely

• tests/ui/macros/rfc-2011-nicer-assert-messages/feature-gate-generic_assert.rs
• tests/ui/macros/rfc-2011-nicer-assert-messages/assert-with-custom-errors-does-not-create-unnecessary-code.rs

By default, unit tests on panic=abort targets will abort the whole process. As these tests rely on #[should_panic] tests, they fail on those targets.

These tests can be made work on panic=abort targets with the additional -Zpanic_abort_tests flag. Both modes (with and without the -Z flag) can be tested using test revisions.

The other, simpler alternative is to mark the tests as needs-unwind to ignore them on panic=abort targets.

I'll send a PR implementing the first alternative.