tests: add more utility functions to store-utils
tests: add more utility functions to store-utils

- adapterRequestFactory: for easier API response stubbing
- assertRelationships: for checking model relationship data

Also

- deeply clone mocked adapter response
  the TwitchSerializer will normalize the response object in-place

streamlink/streamlink-twitch-gui,streamlink/streamlink-twitch-gui,streamlink/streamlink-twitch-gui