Support skipping adt tests by setting ADT=skip

Example to skip the adt tests for some packages and run them in a separate job instead:
http://git.postgresql.org/gitweb/?p=pgapt.git;a=commitdiff;h=34ebf5a322d2c27b5d887fb871d16e7ace68dee

NOTE: If you set ADT variable in your environment make sure to also
allow ADT in sudo's env_keep configuration.

Thanks: Christoph Berg