Dashbio tests refactoring

[shammamah-zz]

Closes #103 (finally!)

Closes #416.

About

• This is a new component
• I am adding a feature to an existing component, or improving an existing feature
• I am closing an issue

Description of changes

• Add new Python test suite that uses dash[testing] for integration tests for all components
• Add initial boilerplate code for unit tests (to be completed in a later PR; right now, the tests only check whether or not the JS elements render)
• Add Percy integration
• Fix some bugs in components (see CHANGELOG updates in d6a8bf5)

Still remaining to complete (in future PRs):

• Add more unit tests for JS components
• Capture canvas elements (they are not included in the DOM, so they are not captured by Percy)

Before merging

• I have gone through the code review checklist
• I have completed all of the steps to take before merging
• I know how to deploy to DDS

[Marc-Andre-Rivet]

A few remaining comments + Byron's comments about the webdriver / require should be addressed.

[Marc-Andre-Rivet]

💃