Database API build tests (#8396)

* Add tests for Database and DatabaseQuery

* Add tests for DatabaseReference

* Add tests for DataEventType

* Add tests for DataSnapshot

* Add tests for MutableData

* Add tests for ServerValue

* Add tests for TransactionResult

* Fix CI for CocoaPods

* Fix CI for SPM and style

* Improve readability

* Fix CI for tvOS

* Refactor type checking

* Fix comments

Add unsurfaced parameter names and shorten async lines

* Fix switch conditional evaluates to a constant

* Fix type checking

* Shorten switch statement

* Add to SPM tests

Author: jrcrespoh

FirebaseDatabase.podspec

@@ -61,6 +61,7 @@ Simplify your iOS development, grow your user base, and monetize more effectivel
     unit_tests.scheme = { :code_coverage => true }
     unit_tests.source_files = [
       'FirebaseDatabase/Tests/Unit/*.[mh]',
+      'FirebaseDatabase/Tests/Unit/Swift/*',
       'FirebaseDatabase/Tests/Helpers/*.[mh]',
       'FirebaseDatabase/Tests/third_party/*.[mh]',
       'SharedTestUtilities/AppCheckFake/*.[mh]',