Gamma tests: redistribute startTestClient wait times (#25)

When we added the active ADS check in front of the client-server
connection check, but we didn't consider where GAMMA test client spends
the majority of its time: `GammaXdsKubernetesTestCase.startTestClient`
had `wait_for_server_channel_ready_timeout` set to 10 minutes before
`wait_for_active_ads_timeout` check existed, but actual increased wait
happens in `wait_for_active_ads_timeout`.

This would cause some of the CSM failures being misclassified as
b/296293582 Timeout establishing ADS stream due to IAM/Workload Identity
propagation delay.

framework/xds_gamma_testcase.py

@@ -127,11 +127,15 @@ def startTestClient(
         # test suites because they only start waiting after already waited for
         # the TD backends to be created and report healthy.
         # In GAMMA, these resources are created asynchronously by Kubernetes.
-        # To compensate for this, we double the timeout for GAMMA tests.
+        # To compensate for this, we double the timeout for the active ADS
+        # stream detection in GAMMA tests. Until the mesh is created,
+        # ADS calls are rejected with "NOT_FOUND: Requested entity was
+        # not found."
         return self._start_test_client(
             server_target,
-            wait_for_server_channel_ready_timeout=datetime.timedelta(
-                minutes=10
-            ),
+            wait_for_active_ads_timeout=datetime.timedelta(minutes=10),
+            # TODO(sergiitk): consider decreasing to 5 minutes, if the majority
+            #     of the wait time spent on waiting ADS.
+            wait_for_server_channel_ready_timeout=datetime.timedelta(minutes=9),
             **kwargs,
         )