Implement safer solution for testing custom settings

[iconix]

PR #22 introduced a setSettingsJsonForTesting method in settings.ts in order to allow for testing of custom, non-production settings. However, exposing a test-only method on Settings like this isn't the best approach, as it could get misused.

Get rid of setSettingsJsonForTesting and implement a DI-like solution so that our test environments can inject a different object than the one in the settings.json production file, as desired.