Fixes the problem introduced by 12b3066aae
which caused nixos/release.nix to return the wrong attributes, while
intending to only affect nixos/lib's runTest.
This also removes callTest from the test options, because callTest is
only ever invoked by all-tests.nix.
Examples: support has to be added to https://github.com/nix-community/nixdoc
'nixdoc --category testers --description "nixpkgs testers" --file ./pkgs/build-support/testers/default.nix'
we will have more testers in the future so they should have their own
location
putting 'testers' in args will also make it simpler to use multiple testers
