Use Map.computeIfAbsent() in SpringClassRule

Replace manual synchronization block in SpringClassRule with Java 8's
Map.computeIfAbsent().

Issue: SPR-12421

spring-test/src/main/java/org/springframework/test/context/junit4/rules/SpringClassRule.java

@@ -210,14 +210,7 @@ public class SpringClassRule implements TestRule {
 	 */
 	static TestContextManager getTestContextManager(Class<?> testClass) {
 		Assert.notNull(testClass, "testClass must not be null");
-		synchronized (testContextManagerCache) {
-			TestContextManager testContextManager = testContextManagerCache.get(testClass);
-			if (testContextManager == null) {
-				testContextManager = new TestContextManager(testClass);
-				testContextManagerCache.put(testClass, testContextManager);
-			}
-			return testContextManager;
-		}
+		return testContextManagerCache.computeIfAbsent(testClass, TestContextManager::new);
 	}