APISIX Dashboard Tests

## Goal

Ensure that the included Dashboard tests pass when Apache APISIX releases a new version.

## Workflow

1. `apache/apisix`: create `release/x.y.z` branch.
2. Update `apache/apisix-dashboard` submodule's commit id to the latest one.
3. Trigger CI in the `apache/apisix` repository to clone the `apache/apisix-dashboard` and run tests against the commit ID.
4. If test cases fail, resolve the issues in `apache/apisix-dashboard` and update the submodule's commit ID.
5. If test cases succeed, continue the release work.

## TODOs

- [x] apache/apisix-dashboard: use `apisix:dev` image to run tests.
- [x] Update `apache/apisix-docker` to push the `apisix:dev` image to DockerHub for every `apache/apisix` commit. (If it's hard to do this in the docker repo, consider putting the logic to apisix main repo). Refer to https://github.com/apache/apisix-docker/blob/master/.github/workflows/apisix_dev_push_docker_hub.yaml Refer to https://github.com/apache/apisix-docker/issues/593

Start by reviewing the release workflow in the issue and the referenced .github/workflows/apisix_dev_push_docker_hub.yaml file in apache/apisix-docker. Check how apache/apisix clones the apache/apisix-dashboard submodule and runs its tests; done means the latest dashboard commit is tested successfully against a new APISIX release and the submodule ID is updated when needed.

Written by the indexing model from the issue text.