Filter out log lines when parsing output of --gtest_list_tests.

This patch makes the Chromium apptest runner resilient to missing apps
(e.g. tracing.mojo) polluting the output of mojo_shell that it tries to
parse.

BUG=450356
TBR=jamesr@chromium.org

Review URL: https://codereview.chromium.org/892933002

Cr-Commit-Position: refs/heads/master@{#314134}

mojo/tools/gtest.py

@@ -87,6 +87,10 @@ def _gtest_list_tests(gtest_list_tests_output):
   """Returns a list of strings formatted as TestSuite.TestFixture from the
   output of running --gtest_list_tests on a GTEST application."""
 
+  # Remove log lines.
+  gtest_list_tests_output = (
+      re.sub("^\[.*\n", "", gtest_list_tests_output, flags=re.MULTILINE))
+
   if not re.match("^(\w*\.\r?\n(  \w*\r?\n)+)+", gtest_list_tests_output):
     raise Exception("Unrecognized --gtest_list_tests output:\n%s" %
                     gtest_list_tests_output)