refs https://github.com/TryGhost/Toolbox/issues/479
- this includes a handful of improvements to get Playwright working on a
local environment including:
- adding `testing-browser` environment so we don't nuke `development`
environments, and makes all the necessary changes to get Ghost to
behave when this is running
- stopped running one global instance of Ghost as this doesn't provide
a clean environment
- copies a few default fixtures that are needed for the new
environment
