Proposed implementation to address WW-5030.

Introduce new junit-portlet plugin.
- Utilizes a copy of org.springframework.mock.web.portlet package from
  Spring 4.3.x, refactored to org.apache.struts2.mock.web.portlet.  Also
  copies one interface from org.apache.struts2.mock.web.portlet.  The
  package-info.java files identify the package renaming.
- Provides test cases for the mock objects.
- Modifies the portlet plugin to use the junit-portlet plugin as a
  dependency.  Adds a small boundary case fix for PortletRequestMap.

plugins/pom.xml

@@ -45,6 +45,7 @@
         <module>jfreechart</module>
         <module>json</module>
         <module>junit</module>
+        <module>junit-portlet</module>
         <module>osgi</module>
         <module>oval</module>
         <module>pell-multipart</module>