Order groups by subsuite and test type (#46816)

As we are trying to not restart browser when there is a test type
switch, doing so will require less browser restarts.

Author: WeizhongX

tools/wptrunner/wptrunner/testloader.py

@@ -553,8 +553,9 @@ def make_queue(self, tests_by_type: TestsByType) -> Tuple[ReadQueue, int]:
         processes = self.process_count(self.kwargs["processes"], len(groups))
         if processes > 1:
             groups.sort(key=lambda group: (
-                # Place groups of the same test type together to minimize
-                # browser restarts.
+                # Place groups of the same subsuite, test type together to
+                # minimize browser restarts.
+                group.subsuite,
                 group.test_type,
                 # Next, run larger groups first to avoid straggler runners. Use
                 # timeout to give slow tests greater relative weight.