Add default test for box sizing

Summary:
Had a mini heart attack thinking I set the default to content box. Wrote this to double check and it passed. Might as well check it in

Technically the default to BoxSizing.h is ContentBox, but in the style we override that. Regardless I switched that around so border box was the default.

Changelog: [Internal]

Differential Revision: D63802722

tests/YGDefaultValuesTest.cpp

@@ -162,3 +162,14 @@ TEST(YogaTest, assert_legacy_stretch_behaviour) {
 
   YGConfigFree(config);
 }
+
+TEST(YogaTest, assert_box_sizing_border_box) {
+  YGConfig* config = YGConfigNew();
+  YGNodeRef root = YGNodeNewWithConfig(config);
+
+  ASSERT_EQ(YGBoxSizingBorderBox, YGNodeStyleGetBoxSizing(root));
+
+  YGNodeFreeRecursive(root);
+
+  YGConfigFree(config);
+}