tests: fix some tests to pass in koji

FAIL: test-engine
=================
Cannot find cgroup mount point: No such file or directory
Assertion 'r >= 0' failed at ../src/test/test-engine.c:46, function main(). Aborting.

FAIL: test-sched-prio
=====================
Cannot find cgroup mount point: No such file or directory
Assertion 'r >= 0' failed at ../src/test/test-sched-prio.c:42, function main(). Aborting.

FAIL: test-bus-cleanup
======================
Assertion 'sd_bus_open_system(&bus) >= 0' failed at ../src/libsystemd/sd-bus/test-bus-cleanup.c:40, function test_bus_open(). Aborting.
after new: refcount 1
man: join tables in systemd-detect-virt(1)

I think it is more readable and nicer if everything is in
one table.

Also, describe what the return value, since it seems awkward to
describe the change in behaviour with --quiet before describing
what the default is.