Fixes AIO Callback testing with Dash-Duo

[MrTeale]

This PR fixes #1933. The AIO components are unable to be tested using Pytest and the Dash-Duo object. The callbacks set via dash.callback (rather than app.callback) are added to a global map that is cleared after the first instance of dash.Dash._setup_server.

To get around this, I'm currently exploring whether it is possible to leave the GLOBAL_CALLBACK_MAP and GLOBAL_CALLBACK_LIST populated rather than clearing them on setup.

Contributor Checklist

• [x] I have broken down my PR scope into the following TODO tasks
  • [x] Explore whether it is as simple as allowing the MAP and LIST to remain populated
  • [] ~~If not, explore whether we can have another MAP as suggested by @T4rk1n in #1933~~
• [x] I have run the tests locally and they passed. (refer to testing section in contributing)
• [x] I have added entry in the CHANGELOG.md

[MrTeale]

Turns out it is possible to not remove them from the GLOBAL_CALLBACK_MAP and clear the GLOBAL_CALLBACK_LIST and everything work fine.

Was there any particular reasoning as to removing them the way they were?