Merge branch 'feature/fix-tests-on-make-all' into 'master'

[cmake][bugfix] Correct handling of the EXCLUDE_FROM_ALL property:

The signature of `dune_add_test`, that takes the `TARGET` from the user
produced excluded targets if added in a directory that has the directory
property `EXCLUDE_FROM_ALL` set. The correct solution IMO is to not use
that property on test subdirectories anymore (it is a relic from the old
testing magic), because we rely on `dune_add_test` to handle exclusion.

See core/dune-common#21 for details.

See merge request !27

dune/istl/paamg/CMakeLists.txt

-add_subdirectory("test" EXCLUDE_FROM_ALL)
+add_subdirectory(test)
 
 #install headers
 install(FILES