In https://github.com/systemd/mkosi/pull/3497, mkosi has started parsing
options passed after the verb as regular mkosi options instead of options
for the invoked command. We adapt to this change by adding '--' as a delimiter
everywhere where required.
This is advanced level stuff that regular contributors don't care
about in the slightest, so move it further down. Also reword the
section a little while we're at it.
The test commands now use mkosi sandbox which always makes sure the
required dependencies for systemd-journal-remote are enabled so no
need to reconfigure meson explicitly anymore.
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.
When running the integration tests downstream, it's useful to be
able to test that a new systemd version doesn't introduce any AVC
denials, so let's add a knob to make that possible.
