bectl(8): Add a test for jail/unjail of numeric BE names

Fixed by r348215, bectl ujail first attempts the trivial fetch of a jid by
passing the first argument to 'ujail' to jail_getid(3) in case a jid/name
have been passed in instead of a BE name. For numerically named BEs, this
was doing the wrong thing: instead of failing to locate the jid specified
and falling back to mountpath search, jail_getid(3) would return the input
as-is.

While here, I've fixed bectl_jail_cleanup which still used a hard-coded pool
name that was overlooked w.r.t. other work that was in-flight around the
same time.

MFC after: 3 days