testing: fix GrpcCleanupRule issue when retrying tests

Fix an issue in GrpcCleanupRule when tests are retried and the
teardown() method is invoked multiple times, causing Stopwatch instance
to throw an IllegalStateException.

fixes #8917.

Author: laurentgo

testing/src/main/java/io/grpc/testing/GrpcCleanupRule.java

@@ -175,6 +175,7 @@ public void evaluate() throws Throwable {
    * Releases all the registered resources.
    */
   private void teardown() {
+    stopwatch.reset();
     stopwatch.start();
 
     if (firstException == null) {