test: add readDirSync tests

fs/unstable_read_dir_test.ts

@@ -1,8 +1,8 @@
 // Copyright 2018-2025 the Deno authors. MIT license.
 
-import { assert, assertEquals, assertRejects } from "@std/assert";
+import { assert, assertEquals, assertRejects, assertThrows } from "@std/assert";
 import { fromFileUrl, join, resolve } from "@std/path";
-import { readDir } from "./unstable_read_dir.ts";
+import { readDir, readDirSync } from "./unstable_read_dir.ts";
 import { NotFound } from "./unstable_errors.js";
 
 const testdataDir = resolve(fromFileUrl(import.meta.url), "../testdata");
@@ -36,3 +36,33 @@ Deno.test("readDir() rejects when the directory does not exist", async () => {
     await readDir("non_existent_dir")[Symbol.asyncIterator]().next();
   }, NotFound);
 });
+
+Deno.test("readDirSync() reads from the directory and its subdirectories", () => {
+  const files = [];
+  for (const e of readDirSync(testdataDir)) {
+    files.push(e);
+  }
+
+  let counter = 0;
+  for (const f of files) {
+    if (f.name === "walk") {
+      assert(f.isDirectory);
+      counter++;
+    }
+  }
+
+  assertEquals(counter, 1);
+});
+
+Deno.test("readDirSync() throws with Error when the path is not a directory", () => {
+  assertThrows(() => {
+    const testFile = join(testdataDir, "0.ts");
+    readDirSync(testFile)[Symbol.iterator]().next();
+  }, Error);
+});
+
+Deno.test("readDirSync() throws with NotFound when a directory does not exist", () => {
+  assertThrows(() => {
+    readDirSync("non_existent_dir")[Symbol.iterator]().next();
+  }, NotFound);
+});