add_*_per target macros don't behave as expected

The issue occurs not only in the add_dune_convergence_test_per_target macro. Also the add_dune_system_test_per_target macro generates too many tests. This is because it doesn't ignore the STATIC section

e.g.

bla = 2
[__STATIC]
GRIDDIM == 2, 3

produces two tests per target although it should only produce one per target.

This is because when having no BASENAME given DO_SOMETHING is always true.