interop-testing: bump client op timeout from 5 to 15 seconds (#9211)

JVM startup costs that happen when dependencies are loaded for the
first time can consume a lot of time (we've occasionally observed
around ~5 seconds of CPU time); this causes frequent test flakes
with xds (google-c2p) when using the current 5 second deadline.
Increasing to 15 seconds should give enough time.

interop-testing/src/main/java/io/grpc/testing/integration/TestServiceClient.java

@@ -619,6 +619,11 @@ public class TestServiceClient {
         return null;
       }
     }
+
+    @Override
+    protected int operationTimeoutMillis() {
+      return 15000;
+    }
   }
 
   private static String validTestCasesHelpText() {