[contrib] validate_failures.py: fix python 3.12 escape sequence warnings

The warnings:
contrib/testsuite-management/validate_failures.py:65: SyntaxWarning: invalid escape sequence '\s'
  _VALID_TEST_RESULTS_REX = re.compile('(%s):\s*(\S+)\s*(.*)'
contrib/testsuite-management/validate_failures.py:77: SyntaxWarning: invalid escape sequence '\.'
  _EXP_LINE_REX = re.compile('^Running (?:.*:)?(.*) \.\.\.\n')

contrib/ChangeLog:
	* testsuite-management/validate_failures.py: Change re.compile()
	function arguments to Python raw strings.