[contrib] validate_failures.py: Add new option --invert_match

This option is used to detect flaky tests that FAILed in the clean
build (or manifest), but PASSed in the current build (or manifest).

The option inverts output logic similar to what "-v/--invert-match"
does for grep.

contrib/ChangeLog:

	* testsuite-management/validate_failures.py (ResultSet.update,)
	(ResultSet.HasTestsuite): New methods.
	(GetResults): Update.
	(ParseSummary, CompareResults, PerformComparison, Main): Handle new
	option --invert_match.