Added check for options.source_dir is none

Author: Jenny Plunkett

tools/test.py

@@ -144,7 +144,7 @@
             if not config:
                 args_error(parser, "argument --test-config contains invalid path or identifier")
         elif not options.app_config:
-            config = TestConfig.get_default_config(options.source_dir, mcu)
+            config = TestConfig.get_default_config(options.source_dir or ['.'], mcu)
         else:
             config = options.app_config