This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
Although it may not be what you were looking for, don't forget that a Servlet is still a Java class. You can put a "main" method in it and call any methods that you want, as long as you can give it the right parameters.
I sometimes use this approach if I need to prform ad-hoc tests of some of the code in a Servlet. It's just a shame how the creators of the Servlet API made it so difficult to "mock" the Request and Response objects.
If you find yourself with methods in a servlet class that need to be tested from a command line, you should take that as a indication that it might be a good idea to refactor the class. If you can create one or more "helper" classes that can be tested outside the servlet environment, it speeds up development and testing.
You can emulate the user data that comes in a ServletRequest with a Map - see the ServletRequest getParameterMap() method. You can use a PrintWriter or OutputStream to get the response.