Fix a test case problem: testSwallowedReadComplete(...) may fail with an AssertionError sometimes. (#10313)

Motivation:

It seems that `testSwallowedReadComplete(...)` may fail with an AssertionError sometimes after my tests. The relevant stack trace is as follows:

```
java.lang.AssertionError: expected:<IdleStateEvent(READER_IDLE, first)> but was:<null>
	at org.junit.Assert.fail(Assert.java:88)
	at org.junit.Assert.failNotEquals(Assert.java:834)
	at org.junit.Assert.assertEquals(Assert.java:118)
	at org.junit.Assert.assertEquals(Assert.java:144)
	at io.netty.handler.flow.FlowControlHandlerTest.testSwallowedReadComplete(FlowControlHandlerTest.java:478)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:498)
	at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
	at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
	at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
	at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
	at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
	at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78)
	at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57)
	at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
	at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
	at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
	at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
	at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
	at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:26)
	at org.junit.internal.runners.statements.RunAfters.evaluate(RunAfters.java:27)
	at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
	at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:89)
	at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:41)
	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:542)
	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:770)
	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:464)
	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:210)

```
Obviously the `readerIdleTime` of `IdleStateHandler` and the thread sleep time before `EmbeddedChannel.runPendingTasks` are both 100ms. And if `userEvents.poll()` happened before `userEvents.add(...)` or no `IdleStateEvent` fired at all, this test case would fail.

Modification:

Sleep for a little more time before running all pending tasks in the `EmbeddedChannel`.

Result:

Fix the problem of small probability of failure.

handler/src/test/java/io/netty/handler/flow/FlowControlHandlerTest.java

@@ -473,7 +473,7 @@ public class FlowControlHandlerTest {
         channel.flushInbound();
         assertNull(channel.readInbound());
 
-        Thread.sleep(delayMillis);
+        Thread.sleep(delayMillis + 20L);
         channel.runPendingTasks();
         assertEquals(IdleStateEvent.FIRST_READER_IDLE_STATE_EVENT, userEvents.poll());
         assertFalse(channel.finish());