main: skip many initialization steps when running in --test mode

main: skip many initialization steps when running in --test mode

Most importantly, don't collect open socket activation fds when in
--test mode. This specifically created a problem because we invoke
pager_open() beforehand (which these days makes copies of the original
stdout/stderr in order to be able to restore them when the pager goes
away) and we might mistakenly the fd copies it creates as socket
activation fds.

Fixes: #6383