[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.