Prepare the code to inherit from chromium's mb.py.

* Don't use 'script' type because it's breaking mb gen.
* Simplify 'raw' type command generation.

Bug: webrtc:13867
Change-Id: I68486c5010c84e41297daafdf11ab63bbca58d4b
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/256862
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Jeremy Leconte <jleconte@google.com>
Cr-Commit-Position: refs/heads/main@{#36351}

tools_webrtc/mb/mb_unittest.py

@@ -522,13 +522,7 @@ class UnitTest(unittest.TestCase):
     self.assertEqual(command, [
         'vpython3',
         '../../tools_webrtc/flags_compatibility.py',
-        'vpython3',
-        '../../testing/test_env.py',
         './base_unittests',
-        '--asan=0',
-        '--lsan=0',
-        '--msan=0',
-        '--tsan=0',
     ])
 
   def test_gen_non_parallel_console_test_launcher(self):