zbd: Fix test scripts

Fix handling of the path to the test-zbd-support script to avoid an
error when the tests are run directly when in fio/t/zbd using the
command "./run-against...".

Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>