Migrate CustomServletPathSampleActuatorTests to JUnit Jupiter

Closes gh-21717

spring-boot-tests/spring-boot-smoke-tests/spring-boot-smoke-test-actuator-custom-security/src/test/java/smoketest/actuator/customsecurity/CustomServletPathSampleActuatorTests.java

 /*
- * Copyright 2012-2019 the original author or authors.
+ * Copyright 2012-2020 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.
@@ -16,23 +16,18 @@
 
 package smoketest.actuator.customsecurity;
 
-import org.junit.runner.RunWith;
-
 import org.springframework.beans.factory.annotation.Autowired;
 import org.springframework.boot.test.context.SpringBootTest;
 import org.springframework.boot.web.server.LocalServerPort;
 import org.springframework.core.env.Environment;
-import org.springframework.test.context.junit4.SpringRunner;
 
 /**
  * Integration tests for actuator endpoints with custom dispatcher servlet path.
  *
  * @author Madhura Bhave
  */
-@RunWith(SpringRunner.class)
 @SpringBootTest(webEnvironment = SpringBootTest.WebEnvironment.RANDOM_PORT,
 		properties = "spring.mvc.servlet.path=/example")
-
 class CustomServletPathSampleActuatorTests extends AbstractSampleActuatorCustomSecurityTests {
 
 	@LocalServerPort