LoadScope scheduler: Sort scopes by number of tests to assign biggest scopes first (#778)

Follow up to # 632.

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Co-authored-by: Jörg Kohlsdorf <joerg@cornershopapp.com>

src/xdist/scheduler/loadscope.py

@@ -350,11 +350,18 @@ class LoadScopeScheduling:
             return
 
         # Determine chunks of work (scopes)
+        unsorted_workqueue = OrderedDict()
         for nodeid in self.collection:
             scope = self._split_scope(nodeid)
-            work_unit = self.workqueue.setdefault(scope, default=OrderedDict())
+            work_unit = unsorted_workqueue.setdefault(scope, default=OrderedDict())
             work_unit[nodeid] = False
 
+        # Insert tests scopes into work queue ordered by number of tests.
+        for scope, nodeids in sorted(
+            unsorted_workqueue.items(), key=lambda item: -len(item[1])
+        ):
+            self.workqueue[scope] = nodeids
+
         # Avoid having more workers than work
         extra_nodes = len(self.nodes) - len(self.workqueue)