tests: Introduce a way to disable options when running in tests (#1095)

In our basic template we used to provide a check based on
`mkTestDerivationFromNvim`. The issue with this check (that is handled
correctly internally) is that some plugins _can't_ be used in the test
environment, for example image.nvim like in #1085.

This commit introduces a new function to generate such checks,
`mkTestDerivationFromNixvimModule`, that wraps a nixvim configuration
instead of a built nvim instance.

Then a configuration can rely on the newly added
`helpers.enableExceptInTests` attribute to disable parts of the
configuration depending if it is evaluated in tests or in a real final
configuration.

Resolves #1085

flake-modules/lib.nix

@@ -13,7 +13,7 @@
       }:
         import ../lib {
           inherit pkgs lib;
-          inherit (config.legacyPackages) makeNixvim;
+          inherit (config.legacyPackages) makeNixvim makeNixvimWithModule;
         }
     )
   );