ci(jest): set maxWorkers to 2

Jest process doesn’t quit after last test completes. This can cause our
tests to fail sometimes, particularly on on Appveyor.

As an attempt to get past this previously we had set `--forceExit` in
our jest cli options, but this doesn't always work either.

Further research suggests that setting ``--maxWorkers=2` could resolve
the issue.

See
 - https://github.com/facebook/jest/issues/1456#issuecomment-424356243
 - https://github.com/facebook/jest/blob/master/docs/Troubleshooting.md

Fix #774, Fix #773

package.json

@@ -35,7 +35,7 @@
     "start-main-dev": "cross-env HOT=1 NODE_ENV=development electron -r babel-register ./app/main.dev",
     "start-renderer-dev": "node --trace-warnings -r babel-register ./node_modules/webpack-serve/lib/cli.js --config internals/webpack/webpack.config.renderer.dev.js",
     "test": "npm run lint && npm run lint-styles && npm run flow && npm run build && npm run test-unit && npm run test-e2e",
-    "test-base": "cross-env NODE_ENV=test BABEL_DISABLE_CACHE=true ELECTRON_DISABLE_SECURITY_WARNINGS=true node --trace-warnings ./node_modules/jest/bin/jest --forceExit",
+    "test-base": "cross-env NODE_ENV=test BABEL_DISABLE_CACHE=true ELECTRON_DISABLE_SECURITY_WARNINGS=true node --trace-warnings ./node_modules/jest/bin/jest --maxWorkers=2",
     "test-unit": "npm run test-base -- --coverage ./test/unit",
     "test-e2e": "npm run test-base -- ./test/e2e",
     "test-ci": "npm run test-e2e && npm run test-unit"

test/e2e/e2e.spec.js

@@ -16,9 +16,9 @@ describe('main window', function spec() {
     await this.app.client.waitUntilWindowLoaded()
   })
 
-  afterAll(() => {
+  afterAll(async () => {
     if (this.app && this.app.isRunning()) {
-      return this.app.stop()
+      await this.app.stop()
     }
   })