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;
 }

lib/helpers.nix

@@ -1,10 +1,11 @@
 {
   lib,
   pkgs,
+  _nixvimTests,
   ...
 }: let
   nixvimTypes = import ./types.nix {inherit lib nixvimOptions;};
-  nixvimUtils = import ./utils.nix {inherit lib;};
+  nixvimUtils = import ./utils.nix {inherit lib _nixvimTests;};
   nixvimOptions = import ./options.nix {inherit lib nixvimTypes nixvimUtils;};
   inherit (import ./to-lua.nix {inherit lib;}) toLuaObject;
 in

lib/utils.nix

@@ -1,9 +1,14 @@
-{lib}:
+{
+  lib,
+  _nixvimTests,
+}:
 with lib; {
   listToUnkeyedAttrs = list:
     builtins.listToAttrs
     (lib.lists.imap0 (idx: lib.nameValuePair "__unkeyed-${toString idx}") list);
 
+  enableExceptInTests = !_nixvimTests;
+
   emptyTable = {"__empty" = null;};
 
   /*