This allows modules that declare their class to be checked.
While that's not most user modules, frameworks can take advantage
of this by setting declaring the module class for their users.
That way, the mistake of importing a module into the wrong hierarchy
can be reported more clearly in some cases.
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.
