[Test] Make test_serialization a medium size test. (#11183)

python/ray/tests/BUILD

@@ -55,6 +55,7 @@ py_test_module_list(
     "test_reconstruction.py",
     "test_reference_counting_2.py",
     "test_reference_counting.py",
+    "test_serialization.py",
     "test_stress.py",
     "test_stress_sharded.py",
     "test_unreconstructable_errors.py",
@@ -90,7 +91,6 @@ py_test_module_list(
     "test_numba.py",
     "test_queue.py",
     "test_ray_init.py",
-    "test_serialization.py",
     "test_tempfile.py",
     "test_webui.py",
   ],