[App Check] Copy SharedTestUtilities for Lite SDK (#11530)

Copied `FIRDateTestUtils` (as `GACDateTestUtils`) and `FIRURLSessionOCMockStub` (as `GACURLSessionOCMockStub`) from the `SharedTestUtilities` directory in the root of the repo into `AppCheck/Tests/Utils/...`. The `SharedTestUtilities` directory won't be available when `AppCheck` is moved into a separate repo.

AppCheck.podspec

@@ -61,8 +61,6 @@ Pod::Spec.new do |s|
     unit_tests.source_files = [
       base_dir + 'Tests/Unit/**/*.[mh]',
       base_dir + 'Tests/Utils/**/*.[mh]',
-      'SharedTestUtilities/Date/*',
-      'SharedTestUtilities/URLSession/*',
     ]
 
     unit_tests.resources = base_dir + 'Tests/Fixture/**/*'