Add test that app renders

[jakecraige]

Add a simple test that verifies the app renders properly when using the test renderer. Add the necessary config to make this work.

• Don't attempt to transform libraries that don't need it. The expo ones aren't totally necessary at this point, but since their libraries are fairly popular and being swallowed up by RN proper, I figure it's worth having them included so that things work if someone ads one of those to their project.
• Temporarily stub out Linking and ScrollView since neither are properly stubbed by default in RN. Reference the open PRs so we can track them and remove once they are shipped.

[jakecraige]

The test example get's a little into the more "example" category than template, but I think it's useful for people new to RN to get a quick idea of what testing with jest and snapshots might look like.

Since it's just test files in the future there could be a way to exclude specific files so that we don't include them when generating the cookie cutter.