[REVIEW] 12023 zfs test zfs_written_property_001_pos can fail

[REVIEW] 12023 zfs test zfs_written_property_001_pos can fail

[Bill Sommerfeld]

Issue: https://www.illumos.org/issues/12023
CR: https://code.illumos.org/c/illumos-gate/+/3446
Diff: 
https://code.illumos.org/~diff/fada23b28987604e008c95373a346e61cd264d9b

I've found that the "zfs_written_property_001_pos" test fails fairly
consistently in my test environment (a bhyve virtual machine which
imports several zfs vdevs from one of the host's zfs pools as virtual
disks).

This test looks at the impact of write I/O on the zfs "written"
property; zfs(8) describes "written" as:

	The amount of space referenced by this dataset,
  	that was written since the previous snapshot (i.e.
	that is not referenced by the previous snapshot).

All of the comparisons to the expected value of "written" are fuzzy in
one way or another; the problem is that they aren't quite loose enough.

Most of them fetch the "parsable" value with zfs get -p and use a
"within_percent" function with a threshhold of 99.5 percent.  I'm
seeing failures where the value was within 99.3 percent, which seems
close enough so I propose to loosen the threshholds from 99.5  to 99.0

The final test does not use "zfs get -p" and does a string compare to
"20.0M" instead of "within_percent".  OpenZFS changed this one to use
"zfs get -p" and "within_percent" a few years back in
https://github.com/openzfs/zfs/pull/9477, and I've incorporated that
fix as well.