WW-3826 solved a problem running JUnit tests on portlet actions that use the struts2-portlet-plugin and struts2-junit-plugin. The solution used Spring's org.springframework.mock.web.portlet package in the spring-test framework. Spring Portlet MVC is no longer supported (SPR-14129) and the package has been removed starting with Spring 5. I'm not able to upgrade to Spring 5 without loosing my unit testing since having both versions of spring-test in my classpath creates many other issues.
I've attached a zipped portlet project for testing (TestStrutsPortlet.zip), console log from a successful test (success.txt) and console log from a failed test (fail.txt). All that needs to change is the spring-version in the POM to recreate the testing.