selftests: ublk: add _ublk_sleep helper for parallel execution

selftests: ublk: add _ublk_sleep helper for parallel execution

Add _ublk_sleep() helper function that uses different sleep times
depending on whether tests run in parallel or sequential mode.

Usage: _ublk_sleep <normal_secs> <parallel_secs>

Export JOBS variable from Makefile so test scripts can detect parallel
execution, and use _ublk_sleep in test_part_02.sh to handle the
partition scan delay (1s normal, 5s parallel).

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