Add downstream tests

[etpinard]

It would be nice to have a workflow that tests ] test Dash either (a) on every DashBase commit or (b) everytime we merge a DashBase PR to master.

[etpinard]

Dropping from the v1 milestone.

I'm thinking it might be better to test Dash with DashBase#master from the Dash repo. That way we could run the Dash integration tests using DashBase#master (on CircleCI) which could be very valuable.