[minor] Add missing 'Test' suffix to CsmObservabilityTestWithInjection (#214)

Test cases must end with the `Test` suffix.

Noticed while reviewing #213.

Author: sergiitk

tests/gamma/csm_observability_with_injection_test.py

@@ -169,7 +169,7 @@ def close(self):
         self.log_stream.close()
 
 
-class CsmObservabilityTestWithInjection(
+class CsmObservabilityTestWithInjectionTest(
     xds_gamma_testcase.GammaXdsKubernetesTestCase
 ):
     metric_client: monitoring_v3.MetricServiceClient