Simplify the `enableExceptInTests` attribute, removing the
`_nixvimTests` argument.
We now do a full re-eval of the nixvim configuration before building the
test, giving us a central place to implement `enableExceptInTests` and
its eventual replacement(s).
This extends support for `enableExceptInTests` to all methods of getting
a nixvim test derivation. Previously, it only worked when using `mkTestDerivationFromNixvimModule`.
In `tests/main.nix`, we avoid the re-eval by doing the initial eval with
a "test mode" lib from the start.