Add more tests

modules/__tests__/missingFile-test.js

@@ -0,0 +1,20 @@
+import request from 'supertest';
+
+import createServer from '../createServer.js';
+
+describe('A request for a non-existent file', () => {
+  let server;
+  beforeEach(() => {
+    server = createServer();
+  });
+
+  it('returns a 404 text error', done => {
+    request(server)
+      .get('/preact@8.4.2/not-here.js')
+      .end((err, res) => {
+        expect(res.statusCode).toBe(404);
+        expect(res.headers['content-type']).toMatch(/\btext\/plain\b/);
+        done();
+      });
+  });
+});