Copy Active Storage test fixture racecar.jpg into Railties fixtures

This change is specifically to support packaging vendors (Fedora) who
test gems in isolation, they don't have the source of the whole Rails
checkout available, and the test files are not packaged with the
`.gem`s. so this is a comprimise so they can still execute this test in
their environment.

Author: zzak

railties/test/application/active_storage/uploads_integration_test.rb

@@ -10,7 +10,7 @@ class UploadsIntegrationTest < ActiveSupport::TestCase
     include Rack::Test::Methods
     include Rails::Dom::Testing::Assertions
 
-    self.file_fixture_path = "#{RAILS_FRAMEWORK_ROOT}/activestorage/test/fixtures/files"
+    self.file_fixture_path = "test/fixtures/files"
 
     def setup
       build_app

railties/test/application/active_storage/validating_service_test.rb

@@ -10,7 +10,7 @@ class ValidatingServiceTest < ActiveSupport::TestCase
     include Rack::Test::Methods
     include Rails::Dom::Testing::Assertions
 
-    self.file_fixture_path = "#{RAILS_FRAMEWORK_ROOT}/activestorage/test/fixtures/files"
+    self.file_fixture_path = "test/fixtures/files"
 
     def setup
       build_app