Prototype of a module with Firestore fakes for testing dependent code in Swift (#7668)

* Firestore testing support prototype

* SwiftPM target

* Comments and formatting

* Move FirebaseFirestoreTestingSupport.podspec to subfolder

* Swift PM updated

* Headers location and imports

* Podspec version update

* Brief readme

* Swift PM FirestoreTestingSupportTests target added

* Formatting

* Move FirebaseFirestoreTestingSupport.podspec back to root

* README updates

* Cleanup podspec

* README

FirebaseFirestoreTestingSupport.podspec

@@ -0,0 +1,58 @@
+Pod::Spec.new do |s|
+  s.name                    = 'FirebaseFirestoreTestingSupport'
+  s.version                 = '1.0.0'
+  s.summary                 = 'Firebase SDKs testing support types and utilities.'
+
+  s.description      = <<-DESC
+  Type declarations and utilities needed for unit testing of the code dependent on Firebase SDKs
+                       DESC
+
+  s.homepage                = 'https://developers.google.com/'
+  s.license                 = { :type => 'Apache', :file => 'LICENSE' }
+  s.authors                 = 'Google, Inc.'
+
+  s.source                  = {
+    :git => 'https://github.com/Firebase/firebase-ios-sdk.git',
+    :tag => 'CocoaPods-' + s.version.to_s
+  }
+
+  ios_deployment_target = '10.0'
+  osx_deployment_target = '10.12'
+  tvos_deployment_target = '10.0'
+  watchos_deployment_target = '6.0'
+
+  s.ios.deployment_target = ios_deployment_target
+  s.osx.deployment_target = osx_deployment_target
+  s.tvos.deployment_target = tvos_deployment_target
+  s.watchos.deployment_target = watchos_deployment_target
+
+  s.cocoapods_version       = '>= 1.4.0'
+  s.prefix_header_file      = false
+  s.requires_arc            = true
+
+  base_dir = 'FirebaseTestingSupport/Firestore/'
+
+  s.source_files = [
+    base_dir + 'Sources/**/*.{m,mm,h}',
+    'Firestore/Source/API/*\+Internal.h'
+  ]
+
+  s.public_header_files = base_dir + '**/*.h'
+
+  s.dependency 'FirebaseFirestore', '~> 7.7'
+
+  s.pod_target_xcconfig = {
+    'GCC_C_LANGUAGE_STANDARD' => 'c99',
+    'OTHER_CFLAGS' => '-fno-autolink',
+    'HEADER_SEARCH_PATHS' =>
+      '"${PODS_TARGET_SRCROOT}" '
+  }
+
+  s.test_spec 'unit' do |unit_tests|
+    unit_tests.scheme = { :code_coverage => true }
+    unit_tests.platforms = {:ios => ios_deployment_target, :osx => osx_deployment_target, :tvos => tvos_deployment_target}
+    unit_tests.source_files = [
+      base_dir + 'Tests/**/*.swift'
+    ]
+  end
+end