test: fix test runner arg regression

Append --node-args to existing list, don't overwrite arg list.

Fixes: https://github.com/nodejs/node/issues/5442
PR-URL: https://github.com/nodejs/node/pull/5446
Reviewed-By: Ben Noorhduis <info@bnoordhuis.nl>
Reviewed-by: Michael Dawson <michael_dawson@ca.ibm.com>

tools/test.py

@@ -709,6 +709,9 @@ class TestRepository(TestSuite):
       (file, pathname, description) = imp.find_module('testcfg', [ self.path ])
       module = imp.load_module('testcfg', file, pathname, description)
       self.config = module.GetConfiguration(context, self.path)
+      if hasattr(self.config, 'additional_flags'):
+        self.config.additional_flags += context.node_args
+      else:
         self.config.additional_flags = context.node_args
     finally:
       if file: