[BUG] dash.testing doesn't work with older versions of dash

[alexcjohnson]

In Dash v1.11 - specifically #1103 - we changed the structure of the redux store used by dash-renderer. This wasn't a breaking change as far as most users are concerned, but dash.testing contains some deeper hooks into this store, and it would be nice to keep these working against older versions of dash, so that you can test an already-deployed app and not worry about the version you're using to test it.

The specific bit that came up in our own work is redux_state_rqs but there may be more. For that one though, we should support both the before and after versions of commit 58eaa96

[gvwilson]

Hi - we are tidying up stale issues and PRs in Plotly's public repositories so that we can focus on things that are most important to our community. If this issue is still a concern, please add a comment letting us know what recent version of our software you've checked it with so that I can reopen it and add it to our backlog. (Please note that we will give priority to reports that include a short reproducible example.) If you'd like to submit a PR, we'd be happy to prioritize a review, and if it's a request for tech support, please post in our community forum. Thank you - @gvwilson