systest: fix race with pool deletion

The second test have pool deletion and object listing wait on the same
semaphore to connect and start. This led to errors sometimes when the
pool was deleted before it could be opened by the listing process. Add
another semaphore so the pool deletion happens only after the listing
has begun.

Fixes: #4147
Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
(cherry picked from commit b0271e390564119e998e18189282252d54f75eb6)
systest: restrict list error acceptance

Only ignore errors after the midway point if the midway_sem_post is
defined.

Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
(cherry picked from commit 5b24a68b6e7d57bac688021b822fb2f73494c3e9)