Add test cases for ImmediateExecutor. (#10060)

Motivation:

ImmediateExecutor needs more test cases.

Modification:

Add several test cases for ImmediateExecutor.

Result:

Improve test coverage slightly.

common/src/test/java/io/netty/util/concurrent/ImmediateExecutorTest.java

@@ -0,0 +1,47 @@
+/*
+ * Copyright 2020 The Netty Project
+ *
+ * The Netty Project licenses this file to you under the Apache License,
+ * version 2.0 (the "License"); you may not use this file except in compliance
+ * with the License. You may obtain a copy of the License at:
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+ * License for the specific language governing permissions and limitations
+ * under the License.
+ */
+
+package io.netty.util.concurrent;
+
+import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertNull;
+import static org.junit.Assert.assertTrue;
+
+import java.util.concurrent.FutureTask;
+
+import org.junit.Test;
+
+public class ImmediateExecutorTest {
+
+    @Test(expected = NullPointerException.class)
+    public void testExecuteNullRunnable() {
+        ImmediateExecutor.INSTANCE.execute(null);
+    }
+
+    @Test
+    public void testExecuteNonNullRunnable() throws Exception {
+        FutureTask<Void> task = new FutureTask<Void>(new Runnable() {
+            @Override
+            public void run() {
+                // NOOP
+            }
+        }, null);
+        ImmediateExecutor.INSTANCE.execute(task);
+        assertTrue(task.isDone());
+        assertFalse(task.isCancelled());
+        assertNull(task.get());
+    }
+}