General improvements to chrome clang plugin test harness

- Passing tests only print "passed" now.
- Failing tests still print the failing command, but in a way that
  should be easier to copy and paste.
- Failing tests now print the diff.
- Bonus feature: filter out lines starting with "DEBUG: " to make printf
  debugging easier. Not that anyone would ever do that.
- Minor python3 fixes and improvements

Change-Id: Ieee080e07cad055d0cf686ebdd0ce24b4e9979ac
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6236972
Reviewed-by: Hans Wennborg <hans@chromium.org>
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1416879}