[cmake][dune_add_test] If target is missing test fails instead of printing "Not Run"

Instead of

Start 4: test_stokes2cni_advection
4/5 Test #4: test_stokes2cni_advection ........***Failed    0.04 sec

it should print the more informative

Start 4: test_stokes2cni_advection
Unable to find required file: test_stokes2cni
4/5 Test #4: test_stokes2cni_advection .........***Not Run   0.00 sec

this can be achieved by setting the test properties to depend on the specified target. See !404 (merged).