test: Implement TEST_PREFER_QEMU and use it in one of the mkosi jobs

test: Implement TEST_PREFER_QEMU and use it in one of the mkosi jobs

We want to make sure the integration tests that don't require qemu
can run successfully both in an nspawn container and in a qemu VM.
So let's add one more knob TEST_PREFER_QEMU=1 to run jobs that normally
require nspawn in qemu instead.

Running these tests in qemu is also possible by not running as root but
that's very implicit so we add an explicit knob instead to make it explicit
that we want to run these in qemu instead of nspawn.