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.

src/test/run-fail/run-unexported-tests.rs

@@ -10,6 +10,7 @@
 
 // error-pattern:runned an unexported test
 // compile-flags:--test
+// check-stdout
 
 extern mod extra;
 

src/test/run-fail/test-fail.rs

@@ -8,6 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+// check-stdout
 // error-pattern:task 'test_foo' failed at
 // compile-flags: --test