tests: fix tests by enabling wrapRc

Since cbd1003d9d I'm able to add _some_
invalid config definitions to modules the tests are using and get no
build error.

For example `extraConfigLua = null;` should produce an invalid type
error, but doesn't.

One less visible change in that commit is the move away from using the
"standalone" wrapper (`makeNixvimWithModule`), which implicitly sets
`wrapRc = true`.

Adding back `wrapRc` to the tests seems to fix the issue, however this
makes me wonder if there's an underlying issue with wrapping/not-wrapping?

Perhaps we've simply uncovered a long-standing eval issue that is masked
over by using `wrapRc`?

lib/tests.nix

@@ -60,6 +60,7 @@ let
               "mkTestDerivationFromNixvimModule: the `dontRun` argument is deprecated. You should use the `test.runNvim` module option instead."
               { config.test.runNvim = !dontRun; }
           ))
+          { wrapRc = true; }
         ];
         extraSpecialArgs = {
           defaultPkgs = pkgs;