Move tmpdir test helper to test helpers file

test/unit/helpers.test.ts

@@ -0,0 +1,14 @@
+import { promises as fs } from "fs"
+import { tmpdir } from "../../test/utils/helpers"
+
+/**
+ * This file is for testing test helpers (not core code).
+ */
+describe("test helpers", () => {
+  it("should return a temp directory", async () => {
+    const testName = "temp-dir"
+    const pathToTempDir = await tmpdir(testName)
+    expect(pathToTempDir).toContain(testName)
+    expect(fs.access(pathToTempDir)).resolves.toStrictEqual(undefined)
+  })
+})