Refactor tests

[bakerkretzmar]

This PR rewrites almost all of Ziggy's tests. It moves us from Mocha to Jest, adds tests for several recent PRs and some edge cases, and cleans up / restructures our test files and the tests themselves. Notably, this also removes the step of running webpack before tests.

I only removed one thing worth mentioning: our tests related to axios. I'm pretty sure they actually weren't running correctly before (the promises weren't returned so they were getting swallowed and no assertions were actually being made), but using both AVA and Jest I couldn't consistently get axios.get(route('name')) to actually work, and axios.get(route('name').url()) isn't worth testing because anything it does is covered by our other tests.

Also sneaking into this PR:

• use a more verbose git command in CI to correctly increment the npm version even when the most recent tag is not annotated
• remove useBuiltIns: false from our Babel config (it's the default value)
• update our Babel/browserslist targets from > 2%, last 2 versions to defaults, not IE 11 (only about a 0.7% difference in global coverage)