Swift Package Manager integration improvements

[danielrbrowne]

There are some improvements that can be made to Package.swift for users integrating using SPM.

• [x] Add a test target for PushNotificationsTests (to mirror the .xcodeproj file set up).
  • [x] Update Nimble dependency from 8.0.1 -> 8.1.2 (I.e. the latest v8.x release).
  • [x] Update OHHTTPStubs dependency from 7.0.0 -> 9.0.0 (Upgrade needed in order to integrate via SPM).
  • [x] Update Cartfile.resolved to match updated dependency versions.