refactor: coerce leaf test modules into derivations

which is much simpler than collecting all the modules and converting
them later on. The result is also easier to work with since it's either
null or a derivation, as opposed to it potentially being a path or a
module.

Author: eljamm

maintainers/types/default.nix

@@ -1,5 +1,7 @@
 {
   lib,
+  pkgs,
+  sources,
   ...
 }@args:
 let

maintainers/types/test.nix

@@ -39,6 +39,9 @@
 */
 {
   lib,
+  pkgs,
+  sources,
+  config,
   ngiTypes,
   ...
 }:
@@ -52,6 +55,64 @@ let
   inherit (ngiTypes)
     problem
     ;
+
+  # TODO: move into `lib.nix`?
+  nixosTest =
+    test:
+    let
+      # Amenities for interactive tests
+      tools = {
+        environment.systemPackages = with pkgs; [
+          vim
+          tmux
+          jq
+        ];
+        # Use kmscon <https://www.freedesktop.org/wiki/Software/kmscon/>
+        # to provide a slightly nicer console.
+        # kmscon allows zooming with [Ctrl] + [+] and [Ctrl] + [-]
+        services.kmscon = {
+          enable = true;
+          autologinUser = "root";
+          fonts = [
+            {
+              name = "Hack";
+              package = pkgs.hack-font;
+            }
+          ];
+        };
+      };
+
+      debugging.interactive.nodes = lib.mapAttrs (_: _: tools) test.nodes;
+
+      args = {
+        imports = [
+          debugging
+          test
+        ];
+        # we need to extend pkgs with ngipkgs, so it can't be read-only
+        node.pkgsReadOnly = false;
+      };
+    in
+    if lib.isDerivation test then
+      lib.lazyDerivation { derivation = test; }
+    else if test == null then
+      null
+    else
+      pkgs.testers.runNixOSTest args;
+
+  callTest =
+    module:
+    if lib.isString module || lib.isPath module then
+      nixosTest (
+        import module {
+          inherit pkgs lib sources;
+          inherit (pkgs) system;
+        }
+      )
+    else if module == null || config.problem != null then
+      null
+    else
+      nixosTest module;
 in
 
 {
@@ -65,7 +126,7 @@ in
       # This is because test nodes are eagerly evaluated to create the
       # driver's `vmStartScripts` (see `nixos/lib/testing/driver.nix` in
       # NixOS).
-      type = with types; nullOr (either deferredModule package);
+      type = with types; nullOr (coercedTo (either deferredModule package) callTest package);
       default = null;
       description = "NixOS test module";
     };