Improve the reliability of some zfsd functional tests

tests/sys/cddl/zfs/tests/zfsd/zfsd_autoreplace_003_pos.ksh
Allow up to 20 seconds for the spare disk to detach after
resilvering completes.

tests/sys/cddl/zfs/tests/zfsd/zfsd_degrade_001_pos.ksh
tests/sys/cddl/zfs/tests/zfsd/zfsd_degrade_002_pos.ksh
Shrink the size of the test pool so checksum errors are more likely
to be generated with a small amount of I/O.

Sponsored by: Spectra Logic Corp