firebase-ios-sdk

Prepare for macOS test support (#2623)

* Add deployment targets for osx

* Fix bundle path to make spec tests work

On iOS, resources are directly within the bundle so the bundlePath and
the resourcePath are the same.

On macOS, resources are in a Contents/Resources subfolder so enumerating
from bundlePath causes us to look for names like
Contents/Resources/offline_spec_test, which doesn't work.

* Fix documentation warnings that show up under the macOS build