Evaluate migration to MockBukkit for Tests

[minoneer]

Evaluate migrating tests to https://github.com/MockBukkit/MockBukkit

• We can more thoroughly test things with Items, Plazers, and Entities that would normally require a running server
• Existing hacky mocks that break frequently with updates can be replaced

• We depend on another framework
• Updates to new versions are not always instant and can break tests