Create a simpletest for the module

I added an apachesolr_rdf.test file with a simpletest. The test is executed for enabled servers of all schemas. First, a new index is created, then filled with one document, then a search is created, the index is searched and results evaluated, then the index is deleted and finally it is tested whether the search still finds data (which it shouldn't).

At the moment, there seem to be some bugs still in the test, as it doesn't manage to create the index.