Add Node.js test suite with a reasonable degree of coverage

[rudokemper]

Feature Request

This PR added some automated tests via Dockerhub, but we should have a basic Node.js test suite as well.

User Stories & Acceptance Criteria

Get tests working with each of the fixtures in tests/fixtures, and the online services.
Add unit tests for any function likely to run into edge cases, and a few integration tests.

• downloadOnlineTiles
• generateStyle
• generateJPG
• generateMBTiles - with fixtures, with an online service, with overlay
• renderTile

Implementation Plan

There might be some tests and fixtures that we can use from mbgl-renderer, albeit that those are largely focused on static image outputs.

[rudokemper]

#23 added the framework but I want to keep this issue open as a reminder to add more tests coverage.