Prepare the code to inherit from chromium's mb.py (3rd attempt).

Basically try to make low_bandwidth_audio_perf_test target work with chromium's mb.

Bug: webrtc:13867
Change-Id: If111e2b0f4f82935062d6321f4d16cc72c77d572
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/256975
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Jeremy Leconte <jleconte@google.com>
Cr-Commit-Position: refs/heads/main@{#36364}

tools_webrtc/mb/mb_unittest.py

@@ -461,7 +461,7 @@ class UnitTest(unittest.TestCase):
          "  'type': 'script',"
          "  'script': '//base/base_unittests_script.py',"
          "}}\n"),
-        '/fake_src/out/Default/base_unittests.runtime_deps':
+        '/fake_src/out/Default/base_unittests_script.runtime_deps':
         ("base_unittests\n"
          "base_unittests_script.py\n"),
     }