Add StringDecoder test (#11496)

Motivation:

There is no test case of `StringDecoder` here

Modification:

Need to add `StringDecoder` test case

Result:

Added test case of `StringDecoder`

Signed-off-by: xingrufei <xingrufei@sogou-inc.com>
Co-authored-by: xingrufei <xingrufei@sogou-inc.com>
Co-authored-by: Norman Maurer <norman_maurer@apple.com>

codec/src/test/java/io/netty/handler/codec/string/StringDecoderTest.java

@@ -0,0 +1,42 @@
+/*
+ * Copyright 2021 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:
+ *
+ *   https://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.handler.codec.string;
+
+import io.netty.buffer.ByteBuf;
+import io.netty.buffer.Unpooled;
+import io.netty.channel.embedded.EmbeddedChannel;
+import io.netty.util.CharsetUtil;
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+import static org.junit.jupiter.api.Assertions.assertNull;
+
+public class StringDecoderTest {
+
+    @Test
+    public void testDecode() {
+        String msg = "abc123";
+        ByteBuf byteBuf = Unpooled.copiedBuffer(msg, CharsetUtil.UTF_8);
+        EmbeddedChannel channel = new EmbeddedChannel(new StringDecoder());
+        assertTrue(channel.writeInbound(byteBuf));
+        String result = channel.readInbound();
+        assertEquals(msg, result);
+        assertNull(channel.readInbound());
+        assertFalse(channel.finish());
+    }
+}

codec/src/test/java/io/netty/handler/codec/string/StringEncoderTest.java

@@ -21,6 +21,8 @@ import io.netty.util.CharsetUtil;
 import org.junit.jupiter.api.Assertions;
 import org.junit.jupiter.api.Test;
 
+import static org.junit.jupiter.api.Assertions.assertFalse;
+
 public class StringEncoderTest {
 
     @Test
@@ -35,5 +37,6 @@ public class StringEncoderTest {
         Assertions.assertArrayEquals(msg.getBytes(CharsetUtil.UTF_8), data);
         Assertions.assertNull(channel.readOutbound());
         buf.release();
+        assertFalse(channel.finish());
     }
 }