selftests: ublk: organize test directories by test ID

Set UBLK_TEST_DIR to ${TMPDIR:-./ublktest-dir}/${TID}.XXXXXX to create
per-test subdirectories organized by test ID. This makes it easier to
identify and debug specific test runs.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>