[bug] Selenium tests contain JS errors due to failed websocket connection

[nemesifier]

Selenium tests contain JS errors due to failed websocket connection.

We need to switch from django's StaticLiveServerTestCase to ChannelsLiveServerTestCase.
I also recommend making the base selenium class in openwisp-utils automatically inherit ChannelsLiveServerTestCase and enforce this across OpenWISP.

We also have to figure out:

• How to automatically assign the debugging port (it seems ChannelsLiveServerTestCase doesn't define the port during class initialization)
• How to use BiDi sessions everywhere (due to Classic sessions being limited, see https://chromium.googlesource.com/chromium/src/+/5b3168e8f8c4487320c654dd761622b89d548514)