ZTS: Remove procfs use from zpool_import_status

procfs might be not mounted on FreeBSD. Plus checking for specific
PID might be not exactly reliable. Check for empty list of jobs
instead.

Premature loop exit can result in failed test and failed cleanup,
failing also some following tests.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de>
Signed-off-by:Alexander Motin <mav@FreeBSD.org>
Sponsored by: iXsystems, Inc.
Closes #11141