Internal fixes for the iOS Explorer test script

Summary:
Some changes are required to this scripts in order to successfully run it on Sandcastle:

- the packager needs to be started before the test
- no xcpretty available, piping to anything else (e.g. `cat`) fails
Closes https://github.com/facebook/react-native/pull/8123

Differential Revision: D3436833

Pulled By: avaly

fbshipit-source-id: 893ac5cba4197cf5bb6b492701171910fd10f99d