[cmake][Release] Allow downstream modules to disable the testing magic

Some downstream modules like pdelab and dumux already abandoned the
current bahaviour of building tests on make test. The testing infrastructure
does throw them error messages during make test though. This commi introduces
the switch DUNE_DISABLE_TEST_MAGIC which may be set by downstream modules
to disable the magic. To see how to switch a downstream module, see:

http://conan2.iwr.uni-heidelberg.de/git/pdelab/dune-pdelab/commit/dc5ef4b39f5be5690457f0bbfe008b1f45d5fd98


(cherry picked from commit ebca3539)
Signed-off-by: Carsten Gräser <graeser@dune-project.org>