[WIP] Add Terminado tests

[GeorgianaElena]

This PR:

• Sets up the testing infra with GHA (hope that's ok)
• Should test the terminado client

Some things to consider:

• The URL class from yarl seems to be escaping the urls if the flag encoded=True is not provided. Because the NotebookSSHServer doesn't use this flag, when trying to connect to http://localhost:12341/a%40b/, where the notebook runs, the url got escaped and got a 404.
• The terminado tests use the NotebookTestBase class

I wanted to open the PR early because I'm a bit stuck with testing writing to stdin (as suggested in https://github.com/yuvipanda/jupyterhub-ssh/issues/13), I don't seem to find the file I'm trying to create anywhere :confused:

Fixes https://github.com/yuvipanda/jupyterhub-ssh/issues/13 Fixes https://github.com/2i2c-org/external/issues/27

[GeorgianaElena]

Thanks @consideRatio! I will refresh the info in this PR in my head and come back with more info on why/where I got blocked with it later today. Hopefully we can figure it out together!