Setup test, break apart entrypoint, & align GHA workflow

backend/__tests__/healthcheck.test.ts

@@ -0,0 +1,19 @@
+import { server } from '../src/app';
+import { describe, expect, it, beforeAll, afterAll } from '@jest/globals';
+import supertest from 'supertest';
+import { setUpHealthEndpoint } from '../src/services/health';
+
+const requestWithSupertest = supertest(server);
+describe('Healthcheck endpoint', () => {
+  beforeAll(async () => {
+    setUpHealthEndpoint(server);
+  });
+  afterAll(async () => {
+    server.close();
+  });
+
+  it('GET /healthcheck should return OK', async () => {
+    const res = await requestWithSupertest.get('/healthcheck');
+    expect(res.status).toEqual(200);
+  });
+});