Add gtest target support for runtest --path.

This is a rough initial version. It makes several assumptions:
 - assumes each test source file is built into module with name equal
to basename of source file
 - assumes tests get installed into /data/nativetest

However, given those assumptions, both:
$ runtest --path <path to directory containing gtest source> and
$ runtest --path <path to single source file>
should work.

Coming soon: gtest host support

Bugs 4563370, 4584339

Change-Id: Ia42aeed7f9ee6402b0ceb7b5ccaaa66ac636fe49

testrunner/adb_interface.py

@@ -263,8 +263,10 @@ class AdbInterface:
     inst_command_string = self._BuildInstrumentationCommand(
         package_name, runner_name, no_window_animation=no_window_animation,
         raw_mode=raw_mode, instrumentation_args=instrumentation_args)
-    command_string = "adb %s shell %s" % (self._target_arg, inst_command_string)
-    return command_string
+    return self.PreviewShellCommand(inst_command_string)
+
+  def PreviewShellCommand(self, cmd):
+    return "adb %s shell %s" % (self._target_arg, cmd)
 
   def _BuildInstrumentationCommand(
       self, package, runner_name, no_window_animation=False, profile=False,