Allow to pass additional shell arguments in `mojo_test`.

mojo/devtools/common/mojo_run

Index: mojo/devtools/common/mojo_run
diff --git a/mojo/devtools/common/mojo_run b/mojo/devtools/common/mojo_run
index a2ac36f4b8e18182655e4ae2379a35a7487efba5..922d7bb4f6f994ec8881571b61fca0b80d94d1ea 100755
--- a/mojo/devtools/common/mojo_run
+++ b/mojo/devtools/common/mojo_run
@@ -10,7 +10,7 @@ import sys
 from devtoolslib import paths
 from devtoolslib import shell_arguments
 
-USAGE = ("mojo_run "
+_USAGE = ("mojo_run "
          "[--args-for=<mojo-app>] "
          "[--content-handlers=<handlers>] "
          "[--enable-external-applications] "
@@ -26,6 +26,11 @@ The value of <handlers> is a comma separated list like:
 text/html,mojo:html_viewer,application/javascript,mojo:js_content_handler
 """)
 
+_DESCRIPTION = """Runner for Mojo applications.
+
+Any arguments not recognized by the script will be passed on as shell arguments.
+"""
+
 
 # Port on which the mojo:debugger http server will be available on the host
 # machine.
@@ -49,7 +54,7 @@ def _ConfigureDebugger(shell):
 def main():
   logging.basicConfig()
 
-  parser = argparse.ArgumentParser(usage=USAGE)
+  parser = argparse.ArgumentParser(usage=_USAGE, description=_DESCRIPTION)
 
   # Arguments allowing to indicate the configuration we are targeting when
   # running within a Chromium-like checkout. These will go away once we have