chore: use default test discovery so npm test works across node builds

`node --test <dir>` treats a directory argument as a module path on some
Node builds (reproduced on v22.23.2), so `npm test` failed before running
any tests. `node --test` with no args discovers the same tests/ files
portably.

package.json

@@ -5,6 +5,6 @@
   "type": "module",
   "description": "A small static personal blog with a mailto-based contact page.",
   "scripts": {
-    "test": "node --test tests/"
+    "test": "node --test"
   }
 }