[cmake][dune_add_test] Set test property to depend on existence of target
If the target was not found in the current state the test fails. With this patch the ctest states "Not Run" and the name of the missing target.
Instead of
Start 4: test_stokes2cni_advection
4/5 Test #4: test_stokes2cni_advection ........***Failed 0.04 sec
it now prints 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
Fixes #100 (closed).
Edited by Timo Koch