Don't return failure for wpt run if not all tests pass.

This isn't very useful unless there is expectation data avaiable, which is
not the common case

tools/wpt/run.py

@@ -420,7 +420,8 @@ def log(x):
 
 def run_single(venv, **kwargs):
     from wptrunner import wptrunner
-    return wptrunner.start(**kwargs)
+    wptrunner.start(**kwargs)
+    return
 
 
 def main():