Make telemetry_unittests.py work on Windows

BUG=422235
R=bartfab@chromium.org

Review URL: https://codereview.chromium.org/647103003

Cr-Commit-Position: refs/heads/master@{#300676}

testing/scripts/telemetry_unittests.py

@@ -18,6 +18,7 @@ def main_run(args):
 
   with common.temporary_file() as tempfile_path:
     rc = common.run_command([
+        sys.executable,
         os.path.join(args.paths['build'], 'scripts', 'tools', 'runit.py'),
         '--show-path',
         os.path.join(args.paths['build'], 'scripts', 'slave', 'runtest.py'),