Let's make this behave more like all the rest of the meson stuff.
This also is the first step to making it a bit more flexible so we
can define integration tests in different ways as will be seen in
the next commits.
CIs set QEMU and nspawn timeouts by themselves which reflect their needs
and possibilities, so let's respect that value, instead of using one
pre-set value which might or might not work for all of them.
Both Ubuntu CI and CentOS CI set these values themselves.
TEST-67-INTEGRITY times out quite often, and when it passes
it does so a few seconds short of the timeout. It's a slow
qemu test, so bump the timeout.
TEST-50-DISSECT has been reported to fail in the same way
on Debian's infrastructure, again narrowly failing or passing
just short of the timeout.
