Uses the xtext-maven-plugin also in the tests project

This commit uses the xtext-maven-plugin also in the tests project on a .builddsl file; this shows that it is possible to use it also for the DSL you're testing, in the same maven run... it was not clear to me that it was possible (see https://www.eclipse.org/forums/index.php/t/825611/ ) so this example could be useful.

Note that I haven't gitignored the builddsl-gen, otherwise you get errors when opening that project in Eclipse (since you don't have the builddsl installed, presumably, in the development IDE).