Pytest webviz fixture

[anders-kiaer]

Dash provides their own pytest fixture, however, still using that fixture, testing different Webviz plugins requires repeating boilerplate code.

Provide a pytest fixture, based on e.g. Dash's dash_duo fixture, to facilitate testing of Webviz plugins.

Some features to consider:

• The Webviz fixture should itself be based on dash_duo.
• Should facilitate creating the app instance, and create an instance of the plugin in a Webviz setting (with given arguments).
• Should be possible to test with CSP settings enabled.
• Should be possible to check if selenium console logs are empty or not.
• Should be possible to check if file resources outside "white-listed" folders are used in portable mode (to ensure the plugin indeed works in a "portable" setting).
• Save screenshots at user defined stages through the test, and optionally compare with stored pictures at given location (visual regression/testing). Stored location could e.g. be a different orphan branch of the github repo?
• Should be possible to automatically check if tour guide element ID's (if defined) actually exists.
• Check that webviz-store decorated functions don't take in pandas objects (which are known to not have well-behaving repr wrt. uniqueness).

Today we mostly test in portable mode. There are some differences, like how assets are loaded. Should faciliate testing both in portable and non-portable mode.