tests/test-derivation: allow tests to be modules

Use `mkTestDerivationFromNixvimModule` instead of `mkTestDerivation`,
allowing "proper" modules to be used instead of plain attr configs.

This is useful for more complex tests that wish to use `config` or
`options` arguments, e.g:

```nix
{config, options, ...}: {
  /* some cool test */
}
```

To allow `tests.dontRun` to be defined on such a test, the module is
allowed to be nested as `module`, e.g:

```nix
{
  tests.dontRun = true;
  module = {config, options, ...}: {
    /* a disabled test */
  };
}
```

Also ended up doing some general cleanup, removing an unused function,
etc.

flake-modules/tests.nix

@@ -3,25 +3,21 @@
   perSystem =
     {
       pkgs,
-      config,
+      pkgsUnfree,
       system,
       helpers,
-      makeNixvimWithModuleUnfree,
       makeNixvimWithModule,
       ...
     }:
     {
       checks = {
         tests = import ../tests {
-          inherit pkgs helpers makeNixvimWithModule;
-          inherit (pkgs) lib;
-          makeNixvim =
-            configuration:
-            makeNixvimWithModuleUnfree {
-              module = {
-                config = configuration;
-              };
-            };
+          inherit
+            pkgs
+            pkgsUnfree
+            helpers
+            makeNixvimWithModule
+            ;
         };
 
         extra-args-tests = import ../tests/extra-args.nix {