Refactor tests to load flake inputs with flake-compat

This makes `nix fmt` just works and we no longer have to override flake
inputs.

tests/treefmt.nix

@@ -0,0 +1,11 @@
+{
+  projectRootFile = "flake.nix";
+  programs = {
+    deadnix = {
+      enable = true;
+      no-lambda-pattern-names = true;
+    };
+    nixfmt.enable = true;
+  };
+  settings.on-unmatched = "info";
+}