Move checks to its own file

checks.nix

@@ -0,0 +1,23 @@
+{ system }:
+{ self, nixpkgs, emacs-overlay, ... }@inputs:
+
+let
+  pkgs = import nixpkgs {
+    inherit system;
+    # we are not using emacs-overlay's flake.nix here,
+    # to avoid unnecessary inputs to be added to flake.lock;
+    # this means we need to import the overlay in a hack-ish way
+    overlays = [ (import emacs-overlay) ];
+  };
+in
+{
+  init-example-el = self.outputs.package.${system} {
+    doomPrivateDir = ./test/doom.d;
+    dependencyOverrides = inputs;
+  };
+  init-example-el-emacsGit = self.outputs.package.${system} {
+    doomPrivateDir = ./test/doom.d;
+    dependencyOverrides = inputs;
+    emacsPackages = with pkgs; emacsPackagesFor emacsGit;
+  };
+}