[contrib] validate_failures.py: Be more stringent in parsing result lines

[contrib] validate_failures.py: Be more stringent in parsing result lines

Before this patch we would identify malformed line
"UNRESOLVEDTest run by tcwg-buildslave on Mon Aug 23 10:17:50 2021"
as an interesting result, only to fail in TestResult:__init__ due
to missing ":" after UNRESOLVED.

This patch makes all places that parse result lines use a single
compiled regex.

contrib/ChangeLog:

	* testsuite-management/validate_failures.py (_VALID_TEST_RESULTS_REX):
	Update.
	(TestResult): Use _VALID_TEST_RESULTS_REX.