#89 Support test-only filter sets in test framework

[Bruce Merry]

Some functionality is made available from the core to filter-sets, and this is the interface boundary at which it is most appropriate to test it (the current place I'm hitting this limitation is in testing the ability to produce aux contexts in combination with GLX_ARB_create_context). It should be possible to specify in the test script a filter set file that will be built and used for the test, but not shipped.