cgroup: don't emit BPF firewall warning when manager is in test mode

Support for BPF might not have been checked, since it's not necessary
in test mode (eg: running offline analysis of units). This causes an
assert:

Assertion '(_error) != 0' failed at src/core/bpf-firewall.c:914, function emit_bpf_firewall_warning(). Aborting.

Export SYSTEMD_LOG_LEVEl=debug in TEST-65-ANALYZE is enough to trigger
this assert while doing an offline analysis of a unit that has some
firewall/network restrictions set.

Skip the warning if the manager is in test mode.

(cherry picked from commit a42232a1)