[Feature Request] - add integration support for dashR

[byronz]

Is your feature request related to a problem? Please describe.
dashR integration tests should use the main dash.testing framework, but there is no current support to make it easier to adapt.

Describe the solution you'd like
The ideal solution should have a harmonized test writing experience in both python and R, the tests for dashR should be the same except the way we host and define the dash app.

Describe alternatives you've considered
the process app runner flavor needs ~1 for the server setup. might need to find a faster way (possible threading solution https://rpy2.readthedocs.io/en/version_2.8.x/) in longer-term.

Additional context

in case we need to support another language testing (e.g. julia), make the tests structure more generic and flexible in terms of

• test selection
• app hosting
• circleci setup

Reference issue