Add tests for exposed identifiers

Author: wooorm

test.js

@@ -2,8 +2,15 @@ import assert from 'node:assert/strict'
 import test from 'node:test'
 import {u} from 'unist-builder'
 import {toString} from './index.js'
+import * as mod from './index.js'
+
+test('toString', () => {
+  assert.deepEqual(
+    Object.keys(mod).sort(),
+    ['toString'],
+    'should expose the public api'
+  )
 
-test('xast-util-to-string', () => {
   assert.deepEqual(
     toString(u('cdata', '<greeting>Hello, world!</greeting>')),
     '<greeting>Hello, world!</greeting>',