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>
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>
Merge remote-tracking branch 'gh/wip-4147'