ZTS: Debug zfs_share_concurrent_shares failure

Update zfs_share_concurrent_shares test case to wait a few seconds
and recheck that the filesystem isn't shared. The intent here is
determine the nature of the error and if it may be a race.

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Umer Saleem <usaleem@ixsystems.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #15379