Fixup gentest-validate when directories have folders

Summary: This reworks `gentest-validate`, to be more resilient to changes like D58307002, which cause [validation failures](https://github.com/facebook/yoga/actions/runs/9653979292/job/26627690870) due to the file traversal code not handling the presence of directories.

Differential Revision: D58987293

.github/actions/setup-js/action.yml

@@ -6,7 +6,7 @@ runs:
     - name: Setup Node environment
       uses: actions/setup-node@v3
       with:
-        node-version: 18.x
+        node-version: 20.x
         cache: yarn
         cache-dependency-path: yarn.lock
       env: