Implement expanded test suite

[rudokemper]

Goal

Now that we have jest as our testing framework, add more tests to cover most of the flow and some likely edge cases.

Screenshots

What I changed

• Add jest coverage reporting.
• Add integration tests for Esri (with overlay), self (MBTiles + GeoJSON source, XYZ dir source), Mapbox, Bing.
• Test generateStyle function.
• Test validateMinMaxValues with some edge cases.
• Streamlined the test fixtures.
• Removed some vestigial code discovered through testing.