start the packager in CI

scripts/test.e2e.ios.js

 function e2e() { //eslint-disable-line
   try {
     if (process.env.CI) {
-      exec.exec(`yarn run start`);
+      exec.execSync(`yarn run start &`);
       exec.execSync(`sleep 5`);
     }
     runDetoxUsingMocha();