test: add basic test for flakeModule functionality

tests/fixtures/flake_parts/flake.nix

@@ -0,0 +1,41 @@
+{
+  outputs = {
+    flake-parts,
+    systems,
+    ...
+  } @ inputs:
+    flake-parts.lib.mkFlake {inherit inputs;} {
+      imports = [
+        "@repo_path@/lib/flakeModule.nix"
+      ];
+      systems = import systems;
+      flake = {};
+      perSystem = _: {
+        ci = {
+          config = {
+            # true is already default, just for testing
+            nixJobsByDefault = true;
+          };
+          pipelines = {
+            "default" = {
+              stages = ["example"];
+              jobs."example" = {
+                stage = "example";
+                script = ["echo hello world"];
+              };
+            };
+            "test".jobs."example" = {
+              stage = ".pre";
+              script = ["echo hello world"];
+            };
+          };
+        };
+      };
+    };
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixpkgs-unstable";
+    flake-parts.url = "github:hercules-ci/flake-parts";
+    systems.url = "github:nix-systems/default-linux";
+  };
+}