Fix run_tests forever mode

tools/run_tests/run_tests.py

@@ -496,7 +496,7 @@ class TestCache(object):
         self.parse(json.loads(f.read()))
 
 
-def _build_and_run(check_cancelled, newline_on_success, travis, cache, xml_report):
+def _build_and_run(check_cancelled, newline_on_success, travis, cache, xml_report=None):
   """Do one pass of building & running tests."""
   # build latest sequentially
   if not jobset.run(build_steps, maxjobs=1,