Allow to pass additional shell arguments in `mojo_test`.

mojo/devtools/common/mojo_test

 #!/usr/bin/env python
 # Copyright 2015 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 """Test runner for Mojo application tests.
 
 The file describing the list of tests has to be a valid Python program that sets
 a |tests| global variable, containing entries of the following form:
 
@@ skipping 13 matching lines @@
    # Optional arguments for the shell.
    "shell-args": ["--some-flag-for-the-shell", "--another-flag"],
  }
 
 The program may use the |target_os| global that will be any of ['android',
 'linux'], indicating the system on which the tests are to be run.
 
 TODO(vtl|msw): Add a way of specifying data dependencies.
 """
 
+_DESCRIPTION = """Runner for Mojo application tests.
+
+Any arguments not recognized by the script will be passed on as shell arguments.
+"""
+
 import argparse
 import sys
 
 from devtoolslib.apptest_runner import run_apptests
 from devtoolslib import shell_arguments
 
 
 def main():
-  parser = argparse.ArgumentParser(description="Test runner for Mojo "
-                                   "application tests.")
+  parser = argparse.ArgumentParser(description=_DESCRIPTION)
   parser.add_argument("test_list_file", type=file,
                       help="a file listing apptests to run")
 
   # Common shell configuration arguments.
   shell_arguments.AddShellArguments(parser)
-  script_args = parser.parse_args()
+  script_args, shell_args = parser.parse_known_args()
 
   try:
-    shell, shell_args = shell_arguments.ConfigureShell(script_args, [])
+    shell, shell_args = shell_arguments.ConfigureShell(script_args, shell_args)
   except shell_arguments.ShellConfigurationException as e:
     print e
     return 1
 
   target_os = 'android' if script_args.android else 'linux'
   test_list_globals = {"target_os": target_os}
   exec script_args.test_list_file in test_list_globals
   apptests_result = run_apptests(shell, shell_args,
                                  test_list_globals["tests"])
   return 0 if apptests_result else 1
 
 if __name__ == '__main__':
   sys.exit(main())