[xDS interop] Limit subsetting test to master branch (grpc#29904)

Author: lidizheng

tools/run_tests/xds_k8s_test_driver/tests/subsetting_test.py

@@ -22,6 +22,7 @@
 
 from framework import xds_k8s_testcase
 from framework import xds_url_map_testcase
+from framework.helpers import skips
 
 flags.adopt_module_key_flags(xds_k8s_testcase)
 
@@ -36,6 +37,13 @@
 
 class SubsettingTest(xds_k8s_testcase.RegularXdsKubernetesTestCase):
 
+    @staticmethod
+    def isSupported(config: skips.TestConfig) -> bool:
+        # Subsetting is an experimental feature where most work is done on the
+        # server-side. We limit it to only run on master branch to save
+        # resources.
+        return config.version_ge('master')
+
     def test_subsetting_basic(self) -> None:
         with self.subTest('00_create_health_check'):
             self.td.create_health_check()