Do not deactive add_test

Summary

The cmake function add_test was deactivated in favor of the dune cmake function dune_add_test. In downstream code, people might want to use other libraries the provide tests, or write their own tests without relying on the dune cmake functions. Thus it was necessary do reactive the standard function add_test. This annoying behavior is removed by this MR. Both ways of defining test are perfectly fine. The dune_add_test macro adds some default compile options and link libraries and handles automatically mpi jobs. But a user can also use the low-level functionality. There is no reason to forbid this.