Move live server testing logic to ucam faas

Description

This task is to move the live server testing logic from account selector to ucam faas testing tools so it can be reused.

Further details

Task list

Acceptance criteria

• ucam_faas.testing exposes live server capabilities
• Implementation allows for running multiple instances/functions, like in account selector (and unlike pytest-flask or pytest-django's implementations)

Links/references

• https://gitlab.developers.cam.ac.uk/uis/devops/iam/activate-account/account-selector/-/blob/main/account_selector/tests/conftest.py?ref_type=heads#L13
• https://gitlab.developers.cam.ac.uk/uis/devops/iam/activate-account/account-selector/-/blob/main/account_selector/tests/live_server.py?ref_type=heads