Add tests for keep-trailing-zeros proposal

[eemeli]

Proposal repo: https://github.com/tc39/proposal-intl-keep-trailing-zeros

The currently proposed spec text won't satisfy all of these tests; I'll be filing a PR incorporating the required changes.

I've created a patched fork of the FormatJS Intl.NumberFormat polyfill to run these tests: https://github.com/eemeli/formatjs