gentest support for position: static + initial test

Summary:
I am about to embark on supporting `position: static` in Yoga. The enum exists already (and is the default position type, lol) but does not actually do anything and just behaves like `position: relative`.

My approach here is to write a bunch of tests to test for the various behaviors of static positions and then develop on Yoga afterwards to get those tests passing. To do this, we need to make a few changes to the gentest files as there is not support for adding `position: static` at the moment:

* Make it so that the gentest code can physically write `YGPositionTypeStatic` if it encounters `position: static` in the style
* Make it so that gentest.js knows that Yoga's default is actually static. This way the code generated in the tests will actually label nodes for non default values
* Explicitly label the position type even when it is not declared in the style prop (with the exception of the default)
* Regenerate all the tests

Additionally I added the first, basic test: making sure insets do nothing on a statically positioned element.

Reviewed By: NickGerleman

Differential Revision: D50437855

fbshipit-source-id: 0e8bbf1c224d477ea4592b7563d0b70d2ffa79c8

gentest/gentest-cpp.js

@@ -127,6 +127,7 @@ CPPEmitter.prototype = Object.create(Emitter.prototype, {
 
   YGPositionTypeAbsolute: {value: 'YGPositionTypeAbsolute'},
   YGPositionTypeRelative: {value: 'YGPositionTypeRelative'},
+  YGPositionTypeStatic: {value: 'YGPositionTypeStatic'},
 
   YGWrapNoWrap: {value: 'YGWrapNoWrap'},
   YGWrapWrap: {value: 'YGWrapWrap'},