Make gentest-validate more graceful of different file organization (#1672)

Summary:
Pull Request resolved: https://github.com/facebook/yoga/pull/1672

`gentest-validate` checks signedsource for generated tests. D58307002 adds a new directory under `java/tests/com/facebook/yoga/`, which [confuses](https://github.com/facebook/yoga/actions/runs/9653979292/job/26627690870) the current directory traversal logic.

This replaces the traversal with `glob`, and makes us skip files without a signature, instead of special-casing the Java directory.

Reviewed By: yungsters

Differential Revision: D58987293

fbshipit-source-id: a5640f5faee4aa40879c266211c5e736a0b077be

.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: