Remove runtest hard dependency on test_defs.xml files.

Previously runtest would fail hard if development/test_runner/test_defs.xml
file was not present, even if it wasn't needed (ie running in --path mode).

This commit skips test_defs parsing if --path is specified, and only parses
development/test_runner/test_defs.xml if its present.

Bug 4556556

Change-Id: Ief2dbbdab5c7e82e5db64414392d6e2a564e815d

testrunner/runtest.py

@@ -195,9 +195,12 @@ class TestRunner(object):
     Raises:
       AbortError: If a fatal error occurred when parsing the tests.
     """
-    core_test_path = os.path.join(self._root_path, self._CORE_TEST_PATH)
     try:
       known_tests = test_defs.TestDefinitions()
+      # only read tests when not in path mode
+      if not self._options.test_path:
+        core_test_path = os.path.join(self._root_path, self._CORE_TEST_PATH)
+        if os.path.isfile(core_test_path):
           known_tests.Parse(core_test_path)
         # read all <android root>/vendor/*/tests/testinfo/test_defs.xml paths
         vendor_tests_pattern = os.path.join(self._root_path,