perf_tests: Update perf_test_runner format.

CL#14740 modified the result format but
the test runner script was never updated.
This fixes the result format and metric check.

Bug: dawn:208
Change-Id: I3a128b91e20ff8629b091ef4bf8fdd6fb616ba0e
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/17921
Reviewed-by: Austin Eng <enga@chromium.org>
Commit-Queue: Bryan Bernhart <bryan.bernhart@intel.com>

scripts/perf_test_runner.py

@@ -115,9 +115,9 @@ def get_results(metric, extra_args=[]):
         print(output)
         sys.exit(3)
 
-    pattern = metric + r'= ([0-9.]+)'
+    pattern = metric + r'.*= ([0-9.]+)'
     m = re.findall(pattern, output)
-    if m is None:
+    if not m:
         print("Did not find the metric '%s' in the test output:" % metric)
         print(output)
         sys.exit(1)