Summary:
Right now there is no way to test fixtures with `auto` widths, heights, or flex basis - even though we expose those functions. I updated gentest to generate those functions. Notably, position and margin (the other auto-able props) already account for this.
I also created `YGAutoTest.html` to test this. Not really testing the capabilities of `auto` here, just if we can create a test about it.
Reviewed By: NickGerleman
Differential Revision: D64125522
