Clean up server-side tests

[Marc-Andre-Rivet]

Server-side tests are built using n-apps for different flavours of the table (e.g. https://github.com/plotly/dash-table/blob/dev/package.json#L25) -- instead, use a single app with routing to simplify writing & running server-side tests for the table.

Alternatively, investigate rewriting the tests with Selenium. We've been having significant issues with Cypress for a while now and those issues don't look like they are going away. Work done in the last year in DCC also greatly improved our Selenium testing environment to the point that maintaining two different sets of testing environments might be moot.