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.js

@@ -654,6 +654,13 @@ function pointValue(e, value) {
       return e.YGAuto;
     case 'undefined':
       return e.YGUndefined;
+    case 'max-content':
+      return e.YGMaxContent;
+    case 'fit-content':
+      return e.YGFitContent;
+    case 'stretch':
+    case '-webkit-fill-available':
+      return e.YGStretch;
     default:
       return value;
   }