core: Skip private /tmp for generators in manager test runs

core: Skip private /tmp for generators in manager test runs

For manager test runs, the generator output paths are located in
/tmp, which means that if we mount a private /tmp for generators,
we lose all the generated units (actually the generators will just
fail because the directories don't exist, but if they did exist,
we'd still lose all the units).

Let's avoid the problem by skipping the private /tmp for manager
test runs. This also avoids any possible privilege issues with
mounting a private /tmp that might happen in this scenario.