fix: deflake SubscriptionPartitionProcessorImplTest (#768)

The test case expects the processor to fail immediately (with deadline of zero) after failing the underlying subscriber. However, the actual failing is done asynchronously. Waiting for a little bit removes the flakiness.

Co-authored-by: Manu Menzella <menzella@google.com>

pubsublite-beam-io/src/test/java/com/google/cloud/pubsublite/beam/SubscriptionPartitionProcessorImplTest.java

@@ -139,7 +139,7 @@ public void subscriberFailureFails() throws Exception {
     processor.start();
     subscriber.fail(new CheckedApiException(Code.OUT_OF_RANGE));
     ApiException e =
-        assertThrows(ApiException.class, () -> processor.waitForCompletion(Duration.ZERO));
+        assertThrows(ApiException.class, () -> processor.waitForCompletion(Duration.millis(10)));
     assertEquals(Code.OUT_OF_RANGE, e.getStatusCode().getCode());
   }