lib: move enableExceptInTests impl to build.test option

Simplify the `enableExceptInTests` attribute, removing the
`_nixvimTests` argument.

We now do a full re-eval of the nixvim configuration before building the
test, giving us a central place to implement `enableExceptInTests` and
its eventual replacement(s).

This extends support for `enableExceptInTests` to all methods of getting
a nixvim test derivation. Previously, it only worked when using `mkTestDerivationFromNixvimModule`.

In `tests/main.nix`, we avoid the re-eval by doing the initial eval with
a "test mode" lib from the start.

lib/default.nix

@@ -2,7 +2,6 @@
   lib,
   flake,
   _isExtended ? false,
-  _nixvimTests ? false,
 }:
 lib.makeExtensible (
   self:
@@ -25,7 +24,7 @@ lib.makeExtensible (
     modules = call ./modules.nix { inherit flake; };
     options = call ./options.nix { };
     plugins = call ./plugins { };
-    utils = call ./utils.nix { inherit _nixvimTests; } // call ./utils.internal.nix { };
+    utils = call ./utils.nix { } // call ./utils.internal.nix { };
 
     # Top-level helper aliases:
     # TODO: deprecate some aliases

lib/tests.nix

@@ -15,7 +15,6 @@ let
       ...
     }:
     let
-      # FIXME: this doesn't support helpers.enableExceptInTests, a context option would be better
       result = nvim.extend {
         config.test = {
           inherit name;
@@ -35,20 +34,13 @@ let
       extraSpecialArgs ? { },
     }:
     let
-      # NOTE: we are importing this just for evalNixvim
-      helpers = self.lib.nixvim.override {
-        # TODO: deprecate helpers.enableExceptInTests,
-        # add a context option e.g. `config.isTest`?
-        _nixvimTests = true;
-      };
-
       systemMod =
         if pkgs == null then
           { nixpkgs.hostPlatform = lib.mkDefault { inherit system; }; }
         else
           { nixpkgs.pkgs = lib.mkDefault pkgs; };
 
-      result = helpers.modules.evalNixvim {
+      result = self.lib.evalNixvim {
         modules = [
           module
           (lib.optionalAttrs (name != null) { test.name = name; })

lib/utils.nix

@@ -1,7 +1,4 @@
-{
-  lib,
-  _nixvimTests,
-}:
+{ lib }:
 rec {
   /**
     Transforms a list to an _"unkeyed"_ attribute set.
@@ -23,15 +20,14 @@ rec {
     builtins.listToAttrs (lib.lists.imap0 (idx: lib.nameValuePair "__unkeyed-${toString idx}") list);
 
   /**
-    Usually `true`, except when nixvim is being evaluated by
-    `mkTestDerivationFromNixvimModule`, where it is `false`.
+    Usually `true`, except within the `build.test` option, where it is `false`.
 
     This can be used to dynamically enable plugins that can't be run in the
     test environment.
   */
   # TODO: replace and deprecate
   # We shouldn't need to use another instance of `lib` when building a test drv
-  enableExceptInTests = !_nixvimTests;
+  enableExceptInTests = true;
 
   /**
     An empty lua table `{ }` that will be included in the final lua configuration.