Fix memory leak regression involving @Async methods

Spring Framework 5.2 M1 introduced a memory leak for applications using
@Async methods. Specifically, in a large test suite with multiple
ApplicationContexts that were closed (e.g., via @DirtiesContext,
@MockBean, or context cache eviction), the JVM process could run out of
memory.

Underlying cause: Due to a missing equals() implementation in Spring's
new AnnotationCandidateClassFilter, CGLIB's static cache of generated
classes indirectly retained references to BeanFactory instances for the
closed ApplicationContexts for the duration of the test suite.

This commit fixes this regression by introducing a proper equals()
implementation in AnnotationCandidateClassFilter. This commit also
introduces corresponding hashCode() and toString() implementations.

Closes gh-23571

spring-test/src/test/java/org/springframework/test/context/async/AsyncMethodsSpringTestContextIntegrationTests.java

@@ -0,0 +1,68 @@
+/*
+ * 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.test.context.async;
+
+import org.junit.jupiter.api.Disabled;
+import org.junit.jupiter.api.RepeatedTest;
+
+import org.springframework.context.annotation.Bean;
+import org.springframework.context.annotation.Configuration;
+import org.springframework.scheduling.annotation.Async;
+import org.springframework.scheduling.annotation.EnableAsync;
+import org.springframework.test.annotation.DirtiesContext;
+import org.springframework.test.context.junit.jupiter.SpringJUnitConfig;
+
+/**
+ * Integration tests for applications using {@link Async @Async} methods with
+ * {@code @DirtiesContext}.
+ *
+ * <p>Execute this test class with {@code -Xmx8M} to verify that there are no
+ * issues with memory leaks as raised in
+ * <a href="https://github.com/spring-projects/spring-framework/issues/23571">gh-23571</a>.
+ *
+ * @author Sam Brannen
+ * @since 5.2
+ */
+@SpringJUnitConfig
+@Disabled("Only meant to be executed manually")
+class AsyncMethodsSpringTestContextIntegrationTests {
+
+	@RepeatedTest(200)
+	@DirtiesContext
+	void test() {
+		// If we don't run out of memory, then this test is a success.
+	}
+
+
+	@Configuration
+	@EnableAsync
+	static class Config {
+
+		@Bean
+		AsyncService asyncService() {
+			return new AsyncService();
+		}
+	}
+
+	static class AsyncService {
+
+		@Async
+		void asyncMethod() {
+		}
+	}
+
+}