run-tests: filter whitespace at end of error diffs
Python 2.6 and 2.7 produce slightly different diff headers.
run-tests: produce '!' mark after running a failing test
This patch fixes a regression recently introduced by a refactoring (see
92a6b16c9186 and about 200 previous changesets from Gregory Szorc). '!' mark
denotes that the test gets failed while testing.