Add python unit test to make sure task in default group doesn't block others. (#20718)

Why are these changes needed?
This PR only adds a unit tests for Python concurrency group.

Related issue number
#20475

python/ray/tests/test_concurrency_group.py

@@ -4,6 +4,7 @@ import sys
 import threading
 import pytest
 import ray
+import time
 
 
 # This tests the methods are executed in the correct eventloop.
@@ -110,5 +111,27 @@ def test_async_methods_in_concurrency_group():
     assert r1 == r2 == r3
 
 
+# This case tests that if blocking task in default group blocks
+# tasks in other groups.
+# See https://github.com/ray-project/ray/issues/20475
+def test_default_concurrency_group_does_not_block_others():
+    @ray.remote(concurrency_groups={"my_group": 1})
+    class AsyncActor:
+        def __init__(self):
+            pass
+
+        async def f1(self):
+            time.sleep(10000)
+            return "never return"
+
+        @ray.method(concurrency_group="my_group")
+        def f2(self):
+            return "ok"
+
+    async_actor = AsyncActor.remote()
+    async_actor.f1.remote()
+    assert "ok" == ray.get(async_actor.f2.remote())
+
+
 if __name__ == "__main__":
     sys.exit(pytest.main(["-v", __file__]))