[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 dune-common#21 (closed) for details.