Fix assertFails not recognising database permission denied error

[mikkelvp]

assertFails does not recognise the permission denied error from database once('value')

example

const app = firebase.initializeTestApp({
  databaseName,
  auth: { uid: 'alice' },
})
app
  .database()
  .ref('secrets/bob/id')
  .once('value')
  .then((snapshot) => {
    console.log('read succeeded')
  })
  .catch((err) => {
    console.log('read failed')
    // current implementation of isPermissionDenied 
    // https://github.com/firebase/firebase-js-sdk/blob/d934f74d6750dc31cca480e126757d53935cefcd/packages/rules-unit-testing/src/api/index.ts#L457
    const isPermissionDenied =
      (err && err.message && err.message.indexOf('PERMISSION_DENIED') >= 0) ||
      (err && err.code === 'permission-denied')
    console.log(`isPermissionDenied ${isPermissionDenied}`)
    console.log(JSON.stringify({ message: err.message, ...err }, null, 2))
  })

output:

read failed
isPermissionDenied false
{
  "message": "permission_denied at /secrets/bob/id: Client doesn't have permission to access the desired data.",
  "code": "PERMISSION_DENIED"
}

[changeset-bot]

🦋 Changeset detected

Latest commit: 878fec2

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@firebase/rules-unit-testing	Patch

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[yuchenshi]

Thanks for the PR! Would you please also add a test for the new case?