test: add JUnit @after import for CallContext cleanup

Author: Surya Srinivasan

server/src/test/java/com/cloud/user/AccountManagerImplTest.java

@@ -59,6 +59,7 @@
 import org.mockito.junit.MockitoJUnitRunner;
 import org.mockito.InjectMocks;
 import org.springframework.beans.factory.NoSuchBeanDefinitionException;
+import org.junit.After;
 
 
 import com.cloud.acl.DomainChecker;