[test] Ignore rename coverage data in execution time.

Bug: v8:6917
Change-Id: I88fdb12c39311531600c0715fff5abe5da051295
Reviewed-on: https://chromium-review.googlesource.com/824271
Commit-Queue: Michał Majewski <majeski@google.com>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50079}

tools/testrunner/local/execution.py

@@ -104,8 +104,9 @@ class TestJob(Job):
   def run(self, context):
     start_time = time.time()
     out = self.cmd.execute()
+    duration = time.time() - start_time
     self._rename_coverage_data(out, context.sancov_dir)
-    return (self.test_id, out, time.time() - start_time)
+    return (self.test_id, out, duration)
 
 
 class Runner(object):