extra: Capture stdout/stderr of tests by default

When tests fail, their stdout and stderr is printed as part of the summary, but
this helps suppress failure messages from #[should_fail] tests and generally
clean up the output of the test runner.
auto merge of #12215 : alexcrichton/rust/test-stdout-capture, r=brson

When tests fail, their stdout and stderr is printed as part of the summary, but
this helps suppress failure messages from #[should_fail] tests and generally
clean up the output of the test runner.