testsuite: Don't check_stats at runtime if not requested
testsuite: Don't check_stats at runtime if not requested

Previously we would call check_stats to check the runtime metrics even
if the test definition hadn't requested it. This would result in an
error since the .stats file doesn't exist.

sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc