Ensure `mkTestDerivationFromNixvimModule` correctly test warnings & assertions.
Also did some minor cleanup:
- Call `failing-tests.nix` using `pkgs.callPackage`
- Replace repetive use of `testBuildFailure` with a wrapper
`mkFailingNixvimTest`
Adds a regression test for #2076. This test ensures that
`extraConfigLua` is used in `finalPackage` and that the test will fail
correctly when running `nvim` results in unexpected output.