Add test support for intrinsic sized nodes

Summary: This diff adds support for intrinsic sizing in generated tests. This is done by importing a testing font called "Ahem" which, as used, has an exact width and height of 10px for each character. Support has been added for C++, Java, and Javascript generated tests.

Differential Revision: D58307002

gentest/gentest.js

@@ -528,6 +528,10 @@ function setupTestTree(
     e.YGNodeInsertChild(parentName, nodeName, index);
   }
 
+  if (node.innerText && node.children.length === 0) {
+    e.YGNodeSetMeasureFunc(nodeName, node.innerText);
+  }
+
   for (let i = 0; i < node.children.length; i++) {
     e.push('');
     const childName = nodeName + '_child' + i;
@@ -723,6 +727,7 @@ function calculateTree(root, parentOffsetLeft, parentOffsetTop) {
         ? child.dataset.experiments.split(' ')
         : [],
       disabled: child.dataset.disabled === 'true',
+      innerText: child.innerText,
     };
 
     const size = getRoundedSize(child);