* FIRAppTests: apple platform flag
* FIRApp: add apple platform to firebaseUserAgent
* Make sure Catalyst app run on macOS is logged as "maccatalyst"
* Comments
* Fix tests to run individually.
Currently these tests rely on other tests to be run first in order for
`kFIRLibraryVersionID` to be assigned. If run alone, these tests crash.
This will prevent that from occurring.
* Style.