Move and refactor Redis test server @Rule

Move the Redis JUnit @Rule so that it can be used with
SessionAutoConfigurationTests. Also refactored the internals a little.

spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/redis/RedisAutoConfigurationTests.java

@@ -95,7 +95,6 @@ public class RedisAutoConfigurationTests {
 	@Test
 	public void testRedisConfigurationWithSentinel() throws Exception {
 		List<String> sentinels = Arrays.asList("127.0.0.1:26379", "127.0.0.1:26380");
-
 		if (isAtLeastOneSentinelAvailable(sentinels)) {
 			load("spring.redis.sentinel.master:mymaster", "spring.redis.sentinel.nodes:"
 					+ StringUtils.collectionToCommaDelimitedString(sentinels));

spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/session/SessionAutoConfigurationTests.java

@@ -17,6 +17,7 @@
 package org.springframework.boot.autoconfigure.session;
 
 import org.junit.After;
+import org.junit.Rule;
 import org.junit.Test;
 import org.springframework.boot.autoconfigure.PropertyPlaceholderAutoConfiguration;
 import org.springframework.boot.autoconfigure.redis.RedisAutoConfiguration;
@@ -25,6 +26,7 @@ import org.springframework.boot.autoconfigure.web.ServerPropertiesAutoConfigurat
 import org.springframework.boot.context.embedded.AnnotationConfigEmbeddedWebApplicationContext;
 import org.springframework.boot.context.embedded.EmbeddedServletContainerFactory;
 import org.springframework.boot.context.embedded.MockEmbeddedServletContainerFactory;
+import org.springframework.boot.redis.RedisTestServer;
 import org.springframework.context.annotation.AnnotationConfigApplicationContext;
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
@@ -39,6 +41,9 @@ import static org.junit.Assert.assertNotNull;
  */
 public class SessionAutoConfigurationTests {
 
+	@Rule
+	public RedisTestServer redis = new RedisTestServer();
+
 	private AnnotationConfigEmbeddedWebApplicationContext context;
 
 	@After