Add data-disabled to test fixtures (#1286)

Summary:
Pull Request resolved: https://github.com/facebook/yoga/pull/1286

This can be marked in fixtures to skip a test without commenting it out. We add one more usage of this.

The same functionality existed (unused) before for `experiments`, which I changed to `data-experiments`.

Formatting of JS tests changed to be closer to what Prettier would output, and to remove usage of `Yoga.UNDEFINED` which doesn't existi and just resolves to `undefined` (this is converted to NaN by the wrapper layer).

Reviewed By: yungsters

Differential Revision: D45723003

fbshipit-source-id: 337af319ab1c1c12047d6579da8c7e63b4f1537a

gentest/gentest-cpp.js

@@ -34,10 +34,15 @@ CPPEmitter.prototype = Object.create(Emitter.prototype, {
     ]);
   }},
 
-  emitTestPrologue:{value:function(name, experiments) {
+  emitTestPrologue:{value:function(name, experiments, disabled) {
     this.push('TEST(YogaTest, ' + name + ') {');
     this.pushIndent();
 
+    if (disabled) {
+      this.push('GTEST_SKIP();');
+      this.push('');
+    }
+
     this.push('const YGConfigRef config = YGConfigNew();')
     for (var i in experiments) {
       this.push('YGConfigSetExperimentalFeatureEnabled(config, YGExperimentalFeature' + experiments[i] +', true);');