Handle disk-full more gracefully in functional tests #23099

If you run the tests with too little disk space in /tmp partition, e.g.

sudo mount -t tmpfs -o size=1M none /tmp
test/functional/test_runner.py

you get a huge log output with a lot of -- Logging error --, with the tests failing one by one, where it's not immediately clear why the tests are failing. It might be useful to exit immediately with a more specific message when even writing to the logs fails, and/or do an early check if the disk space need in the file system where the temporary files go can be estimated.

Beware some systems will auto start testing using /var/tmp/ if they find /tmp/ completely filled up.

That's also an idea. Though, could leave specifying an alternative up to the user. The thing I'm most afraid of here is that someone will end up in a wild goose chase to find what broke the tests and it turns out to be something simple as being out of disk space in /tmp.

good first issue

If anyone's needing this to be done urgently and is willing to commit the necessary energy, I'll be happy to let it go.