add test case for DruidStatService

src/test/java/com/alibaba/druid/stat/spring/UserService.java

@@ -0,0 +1,6 @@
+package com.alibaba.druid.stat.spring;
+
+public interface UserService {
+
+    public void save();
+}

src/test/java/com/alibaba/druid/stat/spring/UserServiceImpl.java

@@ -0,0 +1,12 @@
+package com.alibaba.druid.stat.spring;
+
+import org.springframework.stereotype.Component;
+
+@Component("userService")
+public class UserServiceImpl implements UserService {
+
+    @Override
+    public void save() {
+        // do nothing
+    }
+}