run jest tests

Summary:
Adding support for JS testing. Currently there are two environments tests can run in: node and electron. To select which environment to run a test in, name your test file accordingly `*.node.js` or `*.electron.js` and put it in a `__tests__` folder.

- `yarn test` to run node based tests
- `yarn test-electron` to run electron tests

A basic snapshot test of the empty app is added to make sure the app is rendering as expected. A test for the server is added to make sure when Flipper is started the two servers (secure and insecure) are started and ready to accept connections.

Reviewed By: passy

Differential Revision: D10050212

fbshipit-source-id: 8ef7f931339b43251d9d423886bcaca99ae691e4

src/__tests__/server.electron.js

@@ -0,0 +1,43 @@
+/**
+ * Copyright 2018-present Facebook.
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ * @format
+ */
+
+import Server, {SECURE_PORT, INSECURE_PORT} from '../server.js';
+import LogManager from '../fb-stubs/Logger';
+import path from 'path';
+import os from 'os';
+import fs from 'fs';
+
+let server;
+
+beforeAll(() => {
+  // create config directory, which is usually created by static/index.js
+  const flipperDir = path.join(os.homedir(), '.flipper');
+  if (!fs.existsSync(flipperDir)) {
+    fs.mkdirSync(flipperDir);
+  }
+
+  server = new Server(new LogManager());
+});
+
+test('servers starting at ports', done => {
+  const serversToBeStarted = new Set([SECURE_PORT, INSECURE_PORT]);
+
+  server.addListener('listening', port => {
+    if (!serversToBeStarted.has(port)) {
+      done.fail(Error(`unknown server started at port ${port}`));
+    } else {
+      serversToBeStarted.delete(port);
+    }
+    if (serversToBeStarted.size === 0) {
+      done();
+    }
+  });
+});
+
+afterAll(() => {
+  server.close();
+});