[contrib] validate_failures.py: Support "$tool:" prefix in exp names

This makes it easier to extract the $tool:$exp pair when iterating
over failures/flaky tests, which, in turn, simplifies re-running
testsuite parts that have unexpected failures or passes.

contrib/ChangeLog:

	* testsuite-management/validate_failures.py (_EXP_LINE_FORMAT,)
	(_EXP_LINE_REX, ResultSet): Support "$tool:" prefix in exp names.