[WIP] Introduce @nested tests with testing trait example

This commit introduces a third version of PooledDataBufferTests that
has been migrated from JUnit 4's Parameterized runner support to
JUnit Jupiter features.

- PooledDataBufferNestedTests: based on @nested test classes that all
  implement the same "testing trait" to emulate parameterized support

PooledDataBufferNestedTests has 8 reported tests.

See spring-projectsgh-23451

Author: sbrannen

spring-core/src/test/java/org/springframework/core/io/buffer/PooledDataBufferNestedTests.java

@@ -0,0 +1,98 @@
+/*
+ * Copyright 2002-2019 the original author or authors.
+ *
+ * Licensed 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 org.springframework.core.io.buffer;
+
+import io.netty.buffer.PooledByteBufAllocator;
+import io.netty.buffer.UnpooledByteBufAllocator;
+import org.junit.jupiter.api.Nested;
+import org.junit.jupiter.api.Test;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatIllegalStateException;
+
+/**
+ * @author Arjen Poutsma
+ * @author Sam Brannen
+ */
+public class PooledDataBufferNestedTests {
+
+	@Nested
+	class UnpooledByteBufAllocatorWithPreferDirectTrueTests implements PooledDataBufferTestingTrait {
+
+		@Override
+		public DataBufferFactory createDataBufferFactory() {
+			return new NettyDataBufferFactory(new UnpooledByteBufAllocator(true));
+		}
+	}
+
+	@Nested
+	class UnpooledByteBufAllocatorWithPreferDirectFalseTests implements PooledDataBufferTestingTrait {
+
+		@Override
+		public DataBufferFactory createDataBufferFactory() {
+			return new NettyDataBufferFactory(new UnpooledByteBufAllocator(true));
+		}
+	}
+
+	@Nested
+	class PooledByteBufAllocatorWithPreferDirectTrueTests implements PooledDataBufferTestingTrait {
+
+		@Override
+		public DataBufferFactory createDataBufferFactory() {
+			return new NettyDataBufferFactory(new PooledByteBufAllocator(true));
+		}
+	}
+
+	@Nested
+	class PooledByteBufAllocatorWithPreferDirectFalseTests implements PooledDataBufferTestingTrait {
+
+		@Override
+		public DataBufferFactory createDataBufferFactory() {
+			return new NettyDataBufferFactory(new PooledByteBufAllocator(true));
+		}
+	}
+
+	interface PooledDataBufferTestingTrait {
+
+		DataBufferFactory createDataBufferFactory();
+
+		default PooledDataBuffer createDataBuffer(int capacity) {
+			return (PooledDataBuffer) createDataBufferFactory().allocateBuffer(capacity);
+		}
+
+		@Test
+		default void retainAndRelease() {
+			PooledDataBuffer buffer = createDataBuffer(1);
+			buffer.write((byte) 'a');
+
+			buffer.retain();
+			assertThat(buffer.release()).isFalse();
+			assertThat(buffer.release()).isTrue();
+		}
+
+		@Test
+		default void tooManyReleases() {
+			PooledDataBuffer buffer = createDataBuffer(1);
+			buffer.write((byte) 'a');
+
+			buffer.release();
+			assertThatIllegalStateException().isThrownBy(buffer::release);
+		}
+
+	}
+
+}