Polish LruContextCacheTests

Use AssertJ's extracting() feature instead of ReflectionTestUtils.getField().

Closes spring-projectsgh-31640

spring-test/src/test/java/org/springframework/test/context/cache/LruContextCacheTests.java

@@ -17,16 +17,16 @@
 package org.springframework.test.context.cache;
 
 import java.util.List;
-import java.util.Map;
 
+import org.assertj.core.api.InstanceOfAssertFactories;
 import org.junit.jupiter.api.Test;
 
 import org.springframework.context.ApplicationContext;
 import org.springframework.context.ConfigurableApplicationContext;
 import org.springframework.test.context.MergedContextConfiguration;
-import org.springframework.test.util.ReflectionTestUtils;
 
 import static java.util.Arrays.asList;
+import static org.assertj.core.api.Assertions.as;
 import static org.assertj.core.api.Assertions.assertThat;
 import static org.assertj.core.api.Assertions.assertThatIllegalArgumentException;
 import static org.mockito.Mockito.mock;
@@ -158,17 +158,16 @@ private static MergedContextConfiguration config(Class<?> clazz) {
 	@SuppressWarnings("unchecked")
 	private static void assertCacheContents(DefaultContextCache cache, String... expectedNames) {
 
-		Map<MergedContextConfiguration, ApplicationContext> contextMap =
-				(Map<MergedContextConfiguration, ApplicationContext>) ReflectionTestUtils.getField(cache, "contextMap");
-
-		// @formatter:off
-		List<String> actualNames = contextMap.keySet().stream()
-			.map(cfg -> cfg.getClasses()[0])
-			.map(Class::getSimpleName)
-			.toList();
-		// @formatter:on
-
-		assertThat(actualNames).isEqualTo(asList(expectedNames));
+		assertThat(cache).extracting("contextMap", as(InstanceOfAssertFactories.map(MergedContextConfiguration.class, ApplicationContext.class)))
+				.satisfies((contextMap) -> {
+					// @formatter:off
+					List<String> actualNames = contextMap.keySet().stream()
+							.map(cfg -> cfg.getClasses()[0])
+							.map(Class::getSimpleName)
+							.toList();
+					// @formatter:on
+					assertThat(actualNames).isEqualTo(asList(expectedNames));
+				});
 	}