Merge branch 'feature/new-dune-add-test-features' into 'master'

Implement some new features for dune_add_test

* `COMMANDS` was requested by @tkoch on the mailing list
* `COMPILE_ONLY` is useful for my latest YaspGrid tests, that check the code with and without deprecated interface features.
* `TIMEOUT` sets the timeout for ctest and is need in parallel testing

See merge request !1