Add tests for row-gap and column-gap

Summary:
This adds the fixtures from #1116 and generates tests.

This adds a good amount of coverage, but I plan to follow up with a diff adding a bit more, e.g. for interactions with flex direction of column when we should no-op, etc. I also discovered the current fixtures do not allow testing shorthand props like "gap" without changes.

This also updates the `webdrivers` gem to respond to a break with chromedriver on m1 macs from 4 days ago titusfortner/webdrivers#239.

Reviewed By: yungsters

Differential Revision: D39922413

fbshipit-source-id: dfc7bda894be8dfcb24e25c19a4df0b09a72ce7e