atlas_add_test: add DEPENDS keyword

Add DEPENDS keyword to atlas_add_test to allow the definition of multiple test dependencies:

atlas_add_test( test3
   ...
   DEPENDS test1 test2 )

Note that unlike PROPERTIES DEPENDS this also takes care of pre/post-fixing the dependee names with the package name and _ctest respectively.

Relates to ATLINFR-4473.

TODO:

• Update https://atlassoftwaredocs.web.cern.ch/guides/cmake/#atlas_add_test