fix: create tests dir in start-module skip test for clean checkouts

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

tools/test/start-module.test.mjs

@@ -32,6 +32,7 @@ test("start_module_accepts_when_previous_present", () => {
 
 test("start_module_refuses_skipping", () => {
   const ws = copyFixture("ws-start");
+  mkdirSync(join(ws, "tests"), { recursive: true });
   writeFileSync(join(ws, "tests/m01.rs"), "");
   const r = run("start-module.mjs", ["03", ws], env);
   assert.equal(r.code, 1);