fix(test): testThrottlingBlocking flakyness fix (#1775)

* fix(test): testThrottlingBlocking flakyness fix

When BatcherImpl.sendOutstanding (method that calls callContext.withOption) is called from PushCurrentBatchRunnable, it uses
the batcher's own executor, causing the mockito capture to fail due to be in a different
thread from the one that originated add(). Other times, sendOutstanding will be called from the parent
thread (successful test, in this case from our newFixedThreadPool).
ALthough this is expected behavior, the argument captors do not work
when used in different threads. Mockito.verify() is the recommended way
of dealing with argument captors and happened to be the fix
Internally, verify() relies on thread safe(r) notifiers for when the captors have been interacted with.
When verifying flakiness, 100 runs in two threads were used with
bazel test //gax:com.google.api.gax.batching.BatcherImplTest --runs_per_test=100 --test_filter Throttling --jobs 2

Mockito.verify() implementation:
https://github.com/mockito/mockito/blob/2ded10ec704f2c93648d976031c2520a5a9b84aa/src/main/java/org/mockito/internal/MockitoCore.java#L183

* fix(test): remove unnecessary timeout in verify

gax/src/test/java/com/google/api/gax/batching/BatcherImplTest.java

@@ -887,10 +887,15 @@ public void run() {
       }
 
       try {
-        future.get(100, TimeUnit.MILLISECONDS);
+        future.get(3, TimeUnit.SECONDS);
       } catch (TimeoutException e) {
         assertWithMessage("adding elements to batcher should not be blocked").fail();
       }
+
+      // Mockito recommends using verify() as the ONLY way to interact with Argument
+      // captors - otherwise it may incur in unexpected behaviour
+      Mockito.verify(callContext).withOption(key.capture(), value.capture());
+
       // Verify that throttled time is recorded in ApiCallContext
       assertThat(key.getValue()).isSameInstanceAs(Batcher.THROTTLED_TIME_KEY);
       assertThat(value.getValue()).isAtLeast(throttledTime);