Gentest and initial tests for intrinsic sizing (#1723)

Summary:
Pull Request resolved: https://github.com/facebook/yoga/pull/1723

Adds gentest support for intrinsic sizing keywords and creates an initial batch of somewhat interesting tests for these keywords on `width`, `height`, `min-width`, `min-height`, `max-width`, `max-height`, and `flex-basis`

Reviewed By: NickGerleman

Differential Revision: D64145117

fbshipit-source-id: 1e3e7214fab062ab6f260cfe7bdfaf3c0aca3bf7

gentest/gentest-javascript.js

@@ -15,6 +15,9 @@ function toValueJavascript(value) {
   if (value.match(/^[0-9.e+-]+px$/i)) return parseFloat(value);
   if (value.match(/^[0-9.e+-]+%/i)) return JSON.stringify(value);
   if (value == 'Yoga.AUTO') return '"auto"';
+  if (value == 'max-content') return '"max-content"';
+  if (value == 'fit-content') return '"fit-content"';
+  if (value == 'stretch') return '"stretch"';
   return value;
 }
 
@@ -176,6 +179,10 @@ JavascriptEmitter.prototype = Object.create(Emitter.prototype, {
   YGBoxSizingBorderBox: {value: 'BoxSizing.BorderBox'},
   YGBoxSizingContentBox: {value: 'BoxSizing.ContentBox'},
 
+  YGMaxContent: {value: 'max-content'},
+  YGFitContent: {value: 'fit-content'},
+  YGStretch: {value: 'stretch'},
+
   YGNodeCalculateLayout: {
     value: function (node, dir, _experiments) {
       this.push(node + '.calculateLayout(undefined, undefined, ' + dir + ');');