Fix panic=abort tests on Fuchsia

[tmandry]

#127461 disabled these tests on Fuchsia:

• tests/ui/test-attrs/test-panic-abort-nocapture.rs
• tests/ui/test-attrs/test-panic-abort.rs

They were disabled because they were failing due to a message printed to stderr about the exit code being -1028 (ZX_TASK_RETCODE_EXCEPTION_KILL), coming from this line in libtest (oops).

We can clean up this error message, because it's an expected result of a panic in any unit test, and re-enable the test-panic-abort tests. We just need to add a match arm with #[cfg(target_os = "fuchsia")] that detects the failure code used for fuchsia (-1028) and returns TestResult::TrFailed with no message.

[c6c7]

@rustbot claim