feat: switch from flake-parts & devenv to rensa + add nixtest

tests/render_test.nix

@@ -0,0 +1,30 @@
+{
+  pkgs,
+  ntlib,
+  system,
+  nixlets,
+  ...
+}: {
+  suites."Render Tests" = {
+    pos = __curPos;
+    tests = builtins.map (
+      nixlet: {
+        name = "render ${nixlet.name}";
+        type = "script";
+        script = let
+          output = nixlet.render {inherit system;};
+        in
+          # sh
+          ''
+            ${ntlib.helpers.path [pkgs.gnugrep]}
+            ${ntlib.helpers.scriptHelpers}
+
+            assert "-f ${output}" "should render"
+            assert_file_contains "${output}" "apiVersion"
+            assert_file_contains "${output}" "kubenix/k8s-version"
+            assert_file_contains "${output}" "kubenix/project-name"
+          '';
+      }
+    ) (builtins.attrValues nixlets);
+  };
+}