Misc ZFS test suite fixes

tests/sys/cddl/zfs/tests/history/history_004_pos.ksh
If this test fails, dump the history log.

The last test run had this test fail for the first time in over a
month, without any corresponding changes. Upon inspection, however,
it is only apparent that an additional 17 events occurred than
should have. It seems likely this was caused by external tools
touching the pool. If so, this test will need to be made more
specific.

tests/sys/cddl/zfs/tests/hotspare/hotspare_onoffline_004_neg.ksh
Don't assert that file_trunc is still running a second later. This
is racy; the process may complete successfully before then.
Instead, just assert that the iteration file exists afterwards.

tests/sys/cddl/zfs/tests/zfsd/zfsd.kshlib
wait_for_disk_to_reappear: retry if the device node hasn't arrived.

This will avoid races where this function could return before the
device's corresponding /dev node has been created. Operations made
against the CAM device will work, but not those that access the /dev
node directly, e.g. zpool labelclear.

This change should fix spurious failures from
zfsd_autoreplace_002_pos et al, that appear particularly with debug
kernels.

Submitted by: Will
Sponsored by: Spectra Logic Corp