Add Missing Import

Restored an import that was lost during a backport

Issue gh-4183

Author: jzheaux

web/src/test/java/org/springframework/security/web/authentication/switchuser/SwitchUserFilterTests.java

@@ -17,6 +17,7 @@
 package org.springframework.security.web.authentication.switchuser;
 
 import java.util.ArrayList;
+import java.util.Collection;
 import java.util.List;
 import javax.servlet.FilterChain;