interop-testing: Increase timeout for StressTestClientTest

gaugesShouldBeExported with TSAN doesn't finish in less than 6 seconds
and sometimes takes 12 seconds.

Author: ejona86

interop-testing/src/test/java/io/grpc/testing/integration/StressTestClientTest.java

@@ -44,7 +44,7 @@
 public class StressTestClientTest {
 
   @Rule
-  public final Timeout globalTimeout = Timeout.seconds(10);
+  public final Timeout globalTimeout = Timeout.seconds(15);
 
   @Test
   public void ipv6AddressesShouldBeSupported() {