[Vis Augmenter] Use saved obj imports for test setup

[ohltyler]

Currently, a lot of the test setup (creating dashboards, index patterns, visualizations) for the Vis Augmenter feature is UI-based, and navigates through creation flows which can potentially be flaky on a slow env. Ideally some of these created objects can be stored as saved obj fixtures and imported. There are existing examples in core OSD test suites.

Some of the helper fns to create via UI offer params to make it easy for downstream plugins to use them and create specific dashboards & visualizations for their particular use cases. Will need to explore if it is worth migrating all of this over to static objs which could potentially not scale well if a lot of different objs are needed for different tests

Original comment: #752 (comment)