chore: increase AssertByPollingTest timeout (#1641)

diff --git a/gax-java/gax/src/test/java/com/google/api/gax/batching/AssertByPollingTest.java b/gax-java/gax/src/test/java/com/google/api/gax/batching/AssertByPollingTest.java
index eabf5ed..c64789b 100644
--- a/gax-java/gax/src/test/java/com/google/api/gax/batching/AssertByPollingTest.java
+++ b/gax-java/gax/src/test/java/com/google/api/gax/batching/AssertByPollingTest.java
@@ -79,7 +79,7 @@
           }
         };
 
-    Duration timeout = Duration.ofMillis(100);
+    Duration timeout = Duration.ofMillis(300);
     assertByPolling(timeout, succeedsThirdTime);
     Truth.assertThat(numFailures.get()).isEqualTo(2);
   }