Update to python3 in testing/trigger_scripts/PRESUBMIT.py

This change also includes a fix for a race condition in the xvfb
unit test file, which this change seemed to interact with.

Bug: 1262366
Change-Id: Ic190279c6efa36906123cfa79116b3ac7a072d93
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3270365
Reviewed-by: Dirk Pranke <dpranke@google.com>
Commit-Queue: Matt Reichhoff <mreichhoff@chromium.org>
Cr-Commit-Position: refs/heads/main@{#939920}

testing/trigger_scripts/PRESUBMIT.py

@@ -12,7 +12,11 @@
 
 def CommonChecks(input_api, output_api):
     return input_api.canned_checks.RunUnitTestsInDirectory(
-        input_api, output_api, '.', files_to_check=['.*test.py'])
+        input_api,
+        output_api,
+        '.',
+        files_to_check=['.*test.py'],
+        skip_shebang_check=True)
 
 
 def CheckChangeOnUpload(input_api, output_api):

testing/trigger_scripts/perf_device_trigger_unittest.py

@@ -208,7 +208,7 @@ def test_no_bot_returned(self):
                                    alive_bots=[],
                                    dead_bots=[])
         err_msg = 'Not enough available machines exist in swarming pool'
-        self.assertTrue(err_msg in context.exception.message)
+        self.assertTrue(err_msg in str(context.exception))
 
     def test_previously_healthy_now_dead(self):
         # Test that it swaps out build1 and build2 that are dead

testing/xvfb_test_script.py

@@ -18,15 +18,20 @@
 
 
 def print_signal(sig, *_):
-  print('Signal :{}'.format(sig))
+  # print_function does not guarantee its output won't be interleaved
+  # with other logging elsewhere, but it does guarantee its output
+  # will appear intact. Because the tests parse via starts_with, prefix
+  # with a newline. These tests were previously flaky due to output like
+  # > Signal: 1 <other messages>.
+  print('\nSignal :{}'.format(sig))
 
 
 if __name__ == '__main__':
   signal.signal(signal.SIGTERM, print_signal)
   signal.signal(signal.SIGINT, print_signal)
 
   # test the subprocess display number.
-  print('Display :{}'.format(os.environ.get('DISPLAY', 'None')))
+  print('\nDisplay :{}'.format(os.environ.get('DISPLAY', 'None')))
 
   if len(sys.argv) > 1 and sys.argv[1] == '--sleep':
     time.sleep(2)  # gives process time to receive signal.