ZTS: Fix import_cache_device_replaced

ZTS: Fix import_cache_device_replaced

Allow the 'zpool replace' to run slowly without overwhelming the vdev
queues by setting zfs_scan_vdev_limit=128k.  This limits the number of
concurrent slow IOs which need to be handled.  The net effect is the
test case runs approximately 3x faster putting it well under the 10
minute per-test time limit.

Rename import_cache* test cases to imprt_cachefile*.  Originally
these were renamed due to a maximum tar name limit, this limit was
removed by commit 1dfde3d9b.

Replaced instances of /var/tmp in zpool_import.cfg with $TEST_BASE_DIR.

Reviewed-by: bunder2015 <omfgbunder@gmail.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #7765
Closes #7802