Support running a fixture only once per entire execution

For some cases, we might want the setup of the test to run only once, even when something is multi processed, for example:

We write our end-to-end tests in python using pytest, this means that one of our fixtures takes care of bringing up the entire environment of the test.

One of the current problems we are facing with parallelizing our tests is that we can't use xdist, since it runs a session-scoped fixture for every instance.

A way to run a fixture only once, no matter how many processes are there would be very beneficial for such cases where the setup should only run once in a background process.

Start by reviewing how pytest session-scoped fixtures behave when pytest-xdist runs tests in multiple processes. Compare the current xdist fixture lifecycle with the requested single background setup, including how processes would coordinate and share its result. Done means a fixture can be configured to initialize once across the entire execution while parallel workers continue using it safely.