Allow adding JARs to XWiki's WEB-INF/lib when testing with Docker

There are some cases where the module being tested cannot be fully deployed as an extension into a running XWiki and it needs to deploy one or several JARs into WEB-INF/lib. These are cases that we need to fix but this is about offering a way around the current limitations.

One such example is when an extension includes a Hibernate HBM file. Right now it seems only HBM files located in JARs in WEB-INF/lib are supported.