* Move test helpers into common modules
* helpers.js gets all the helper functions
* setup.js gets the common Jest setup (serializer, expect extensions)
* Exclude test from coverage testing
* @ylemkimon's comments: parsing -> building, settings || defaultSettings
* Default argument for settings
* Fix lint errors
* @ylemklemon's comment: use buildAndSetResult
* Use template literals
* Stop throwing ParseError when throwOnError is false
`render`, `renderToString`, etc. now catch ParseError and render it to the
raw LaTeX (with proper escaping) and a hover title with the error.
Along the way:
* Use new `katex.__renderToDomTree` in katex-spec's `_getBuilt`.
(This was necessary to get the new error handling in `_getBuilt`.)
* Fix jest results which must always be functions, not strings.
* fix lint
* Fix flow error
Leave error type unspecified, as we check it with instanceof.
* Update katex-spec.js
