Add functions spm unit and build tests (#8499)

* Update FunctionsAPITests.swift with watchos and Task

* Add watchos deployment target to Functions podspec

* Add spm Functions unit and build tests

* Update travis.yml for functions and watchos

* Disable failing travis test, passes locally, wait for Xcode 12.5 on GHA

* Changelog

* Delete unused files

Co-authored-by: Paul Beusterien <paulbeusterien@google.com>

FirebaseFunctions.podspec

@@ -18,6 +18,7 @@ Cloud Functions for Firebase.
   s.ios.deployment_target = '10.0'
   s.osx.deployment_target = '10.12'
   s.tvos.deployment_target = '10.0'
+  s.watchos.deployment_target = '6.0'
 
   s.cocoapods_version = '>= 1.4.0'
   s.prefix_header_file = false