[contrib] validate_failures.py: Add "--expiry_date YYYYMMDD" option

This option sets "today" date to compare expiration entries against.
Setting expiration date into the future allows re-detection of flaky
tests and creating fresh entries for them before the current flaky
entries expire.

contrib/ChangeLog:

	* testsuite-management/validate_failures.py (TestResult): Update.
	(Main): Handle new option "--expiry_date YYYYMMDD".