The file test/HelloWorld.class is not a web application. A web application is a directory or zip file with the following contents:
WEB-INF\web.xml WEB-INF\classes WEB-INF\lib JSP pages HTML pages images etc.
You should find a good tutorial on web applications to learn more about this.
My best guess is HelloWorld is a servlet in the test package. If you configure this servlet with a name of HelloWorld and a class of test.HelloWorld in the default web application then you should be able to invoke it with the URL:
By using a directory structure of "/test" you have deployed this servlet in the 'test' web application not the default web application. Since it is deployed in the 'test' web application which has a context path of "/test" then you must invoke anything in the 'test' web application with URLs that begin with "/test".
The easiest way to add a servlet to the default app is to place it in /default-app/WEB-INF/classes and configure it in /default-app/WEB-INF/web.xml.