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/pom.xml

@@ -246,6 +246,11 @@
 			<artifactId>spring-context-support</artifactId>
 			<scope>test</scope>
 		</dependency>
+		<dependency>
+			<groupId>org.springframework.data</groupId>
+			<artifactId>spring-data-redis</artifactId>
+			<scope>test</scope>
+		</dependency>
 	</dependencies>
 	<build>
 		<plugins>