Only test things which aren't derivations

[Chris Warburton]

release.nix should pass into test.nix the set of non-test things it
already has. test.nix should check that these are all derivations, then
it should check that all of the customNames which aren't in the given
set are not derivations (i.e. nothing's been left out).

It should add a test derivation where appropriate which 'exercise' the
non-derivation things. Actual testing should be done by those things,
e.g. with assertions or withDeps, etc. The job of test.nix is just to
add them to release.nix in a way that triggers them.