[tests] Add basic selenium tests to give us confidence on dependabot PRs

[nemesifier]

We need some basic selenium browser tests to give us more confidence in merging dependabots PRs.

I think writing basic tests for these 2 features would be enough for now:

• https://openwisp-radius.readthedocs.io/en/latest/user/importing_users.html
• https://openwisp-radius.readthedocs.io/en/latest/user/generating_users.html