Make one macro approach work when lacking static variations

With these changes it it possible to use the one macro approach with add_dune_system_test although there are no static variations (issue #28 (closed) ).

Furthermore it implements full support for the one macro approach for generic numbering in the absence of the "exec_suffix" keyword. The targets are numbered and now connected to the right ini file. This avoids lengthy names for executables with the disadvantage of having not self-explanatory names. Self-explanatory names might be not that important (e.g. targets get created on a server by build-bot).