selftests: ublk: derive TID automatically from script name

selftests: ublk: derive TID automatically from script name

Add automatic TID derivation in test_common.sh based on the script
filename. The TID is extracted by stripping the "test_" prefix and
".sh" suffix from the script name (e.g., test_loop_01.sh -> loop_01).

This removes the need for each test script to manually define TID,
reducing boilerplate and preventing potential mismatches between
the script name and TID. Scripts can still override TID after
sourcing test_common.sh if needed.

Reviewed-by: Caleb Sander Mateos <csander@purestorage.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>