Add tests for FlushStrategy (java-native-access#229)

Motivation:

2423c2f added the FlushStrategy API but did not add extra tests

Modifications:

Add unit tests for the two factory methods of FlushStrategy

Result:

Better test coverage

src/test/java/io/netty/incubator/codec/quic/FlushStrategyTest.java

@@ -0,0 +1,38 @@
+/*
+ * 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.incubator.codec.quic;
+
+import org.junit.Test;
+
+import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertTrue;
+
+public class FlushStrategyTest {
+
+    @Test
+    public void testAfterNumBytes() {
+        FlushStrategy strategy = FlushStrategy.afterNumBytes(10);
+        assertFalse(strategy.shouldFlushNow(1, 10));
+        assertTrue(strategy.shouldFlushNow(1, 11));
+    }
+
+    @Test
+    public void testAfterNumPackets() {
+        FlushStrategy strategy = FlushStrategy.afterNumPackets(10);
+        assertFalse(strategy.shouldFlushNow(10, 10));
+        assertTrue(strategy.shouldFlushNow(11, 11));
+    }
+}