ZFS: multiple fixes to the zpool_import tests

• Don't create a UFS mountpoint just to store some temporary files. The tests should always be executed with a sufficiently large TMPDIR. Creating the UFS mountpoint is not only unneccessary, but it slowed zpool_import_missing_002_pos greatly, because that test moves large files between TMPDIR and the UFS mountpoint. This change also allows many of the tests to be executed with just a single test disk, instead of two.

• Move zpool_import_missing_002_pos's backup device dir from / to $PWD to prevent cross-device moves. On my system, these two changes improved that test's speed by 39x. It should also prevent ENOSPC errors seen in CI.

• If insufficient disks are available, don't try to partition one of them. Just rely on Kyua to skip the test. Users who care will configure Kyua with sufficient disks.

MFC after: 2 weeks
Sponsored by: Axcient