Add golden tests for onboarding #2520

Job	Run time
flutter_packages (packages/rx_bloc_list)	3m 3s
cli_packages (packages/rx_bloc_cli)	2m 32s
examples (examples/booking_app)	2m 36s
flutter_packages (packages/flutter_rx_bloc)	3m 14s
examples (examples/counter)	1m 48s
examples (examples/division)	1m 37s
packages (packages/rx_bloc)	2m 3s
packages (packages/rx_bloc_test)	2m 23s
examples (examples/favorites_advanced/rx_bloc_favorites_advanced)	2m 4s
packages (packages/rx_bloc_generator)	3m 30s
examples (examples/todoapp)	2m 59s
build	1s
	27m 50s

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