XAR Plugin unit test fail when executed in IntelliJ IDEA

The reason is that in IntelliJ IDEA the maven basedir is the top level project (while in Eclipse for ex it's the module's directory and not hte top level one I think).

In addition:

• XARMojoTest doesn't verify that the Maven XAR custom packaging works
• XARMojoTest is isolated from the other integration tests located in the xwiki-commons-tool-xar-integration-tests module

Thus rewrite the XARMojoTest inside the xwiki-commons-tool-xar-integration-tests module