selftests: ublk: add group-based test targets

selftests: ublk: add group-based test targets

Add convenient Makefile targets for running specific test groups:
- run_generic, run_batch, run_null, run_loop, run_stripe, run_stress, etc.
- run_all for running all tests

Test groups are auto-detected from TEST_PROGS using pattern matching
(test_<group>_<num>.sh -> group), and targets are generated dynamically
using define/eval templates.

Supports parallel execution via JOBS variable:
- JOBS=1 (default): sequential with kselftest TAP output
- JOBS>1: parallel execution with xargs -P

Usage examples:
  make run_null           # Sequential execution
  make run_stress JOBS=4  # Parallel with 4 jobs
  make run_all JOBS=8     # Run all tests with 8 parallel jobs

With JOBS=8, running time of `make run_all` is reduced to 2m2s from 6m5s
in my test VM.

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