Add `Beacon.Test` and `Beacon.Test.Fixtures`

[leandrocp]

Breaking Changes

• Remove config :skip_boot? in favor of :mode which can be :auto or :manual (defaults to :auto)

Enhancements

• Add Beacon.Test that provides testing utilities to use on host apps
• Add Beacon.Test.Fixtures to expose fixtures to seed test data, the same used by Beacon itself
• Reload modules synchronously on manual mode
• Leverage :manual mode during boot to avoid unnecessary calls to Tailwind compiler, speeding up the whole process

Documentation

• Added "Testing" recipe to demonstrate usage of Beacon.Test