test: Add option to save in progress test journals to /tmp

test: Add option to save in progress test journals to /tmp

The journal isn't the best at being fast, especially when writing
to disk and not to memory, which can cause integration tests to
grind to a halt on beefy systems due to all the systemd-journal-remote
instances not being able to write journal entries to disk fast enough.

Let's introduce an option to allow writing in progress test journals
to use /tmp which can be used on beefy systems with lots of memory to
speed things up.