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

lib/default.nix

@@ -1,12 +1,13 @@
 # Args probably only needs pkgs and lib
 {
   makeNixvim,
+  makeNixvimWithModule,
   pkgs,
   ...
 } @ args: {
   # Add all exported modules here
   check = import ../tests/test-derivation.nix {
-    inherit makeNixvim pkgs;
+    inherit makeNixvim makeNixvimWithModule pkgs;
   };
   helpers = import ./helpers.nix args;
 }