This test wrong at many points:

• test returns before calling ParentServlet, which is responsible for ChildServlet registration;
• webclient calls wrong URLs (/ is redundant and don't match with mappings);
• mapping for ChildServlet isn't set (dynamically)
• fail(..) statement prevents execution of meaningful part of the test;
• and finally it wouldn't work because programmatic registration is impossible after ServletContext initialization is finished.