Use 100MB pool for filetest_001_pos.ksh checksum test

Use 100MB pool for filetest_001_pos.ksh checksum test

As part of its tests, filetest_001_pos.ksh wipes the entire vdev to
create checksum errors.  This patch uses the setup/cleanup scripts from
the scrub_mirror test to create a custom 100MB pool, rather than
using the entire device size that is passed into zfs-tests.sh
(which defaults to 2GB).  This speeds up the buildbot tests, and also
makes it possible for someone to use real disks (say, 1TB) without the
test taking an insanely long amount of time.