Auto merge of #4970 - Gilnaa:master, r=alexcrichton

Relaxed testing of libtest outputs.

This PR relaxes some checks on libtest due to upcoming changes by this PR:
https://github.com/rust-lang/rust/pull/46450

Summary of expected changes:

- Crashing benchmarks are now caught and handled just like in tests, and should not crash the harness. The old cargo test checked that the panic text was printed in stderr. In the patched behaviour, the text is printed to stdout.
With the advice of @alexcrichton, I changed test to check for the text in either stream in order to account of the transition period.