OpenZFS 9245 - zfs-test failures: slog_013_pos and slog_014_pos

OpenZFS 9245 - zfs-test failures: slog_013_pos and slog_014_pos

Test 13 would fail because of attempts to zpool destroy -f a pool that
was still busy. Changed those calls to destroy_pool which does a retry
loop, and the problem is no longer reproducible. Also removed some non
functional code in the test which is why it was originally commented out
by placing it after the call to log_pass.

Test 14 would fail because sometimes the check for a degraded pool would
complete before the pool had changed state. Changed the logic to check
in a loop with a timeout and the problem is no longer reproducible.

Authored by: John Wren Kennedy <john.kennedy@delphix.com>
Reviewed by: Matt Ahrens <matt@delphix.com>
Reviewed by: Chris Williamson <chris.williamson@delphix.com>
Reviewed by: Yuri Pankov <yuripv@yuripv.net>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Approved by: Dan McDonald <danmcd@joyent.com>
Ported-by: Brian Behlendorf <behlendorf1@llnl.gov>
Porting Notes:
* Re-enabled slog_013_pos.ksh

OpenZFS-issue: https://illumos.org/issues/9245
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/8f323b5
Closes #7585