[FLINK-23045][tests] Harden RunnablesTest by not relying on timeout

This closes apache#16261.
This commit hardens the RunnablesTest.testExecutorService_uncaughtExceptionHandler to not rely
on timeouts to check whether the uncaught exception handler was called.

flink-runtime/src/test/java/org/apache/flink/runtime/util/RunnablesTest.java

@@ -55,9 +55,9 @@ public void testExecutorService_uncaughtExceptionHandler() throws InterruptedExc
                 () -> {
                     throw new RuntimeException("foo");
                 });
-        Assert.assertTrue(
-                "Expected handler to be called.",
-                handlerCalled.await(TIMEOUT_MS, TimeUnit.MILLISECONDS));
+
+        // expect handler to be called
+        handlerCalled.await();
     }
 
     @Test