test: move pages tests out of Next.js routes

src/__tests__/pages/api/healthcheck.test.js

@@ -0,0 +1,16 @@
+import { describe, expect, it } from "vitest";
+
+import createMockRes from "test-utils/create-mock-res";
+
+import handler from "pages/api/healthcheck";
+
+describe("pages/api/healthcheck", () => {
+  it("returns 'up'", () => {
+    const req = {};
+    const res = createMockRes();
+
+    handler(req, res);
+
+    expect(res.body).toBe("up");
+  });
+});