Suppress successful test output from script/test to make it easier to find failures.

I didn't see any quiet options for swift test, but I can look again or manually | grep -v for all output that doesn't mention errors.

Should I still output the end summary if all tests succeed, or should the output be completely blank if everything succeeds?

I personally prefer not outputting anything on complete success.

I could also add a --verbose flag to not filter anything out (or a `--quiet flag to perform the filtering with no flag outputting the existing output).