selftests: ublk: improve test usability

selftests: ublk: improve test usability

Add UBLK_TEST_QUIET, so we can print test result(PASS/SKIP/FAIL) only.

Also always run from test script's current directory, then the same test
script can be started from other work directory.

This way helps a lot to reuse this test source code and scripts for
other projects(liburing, blktests, ...)

Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250303124324.3563605-12-ming.lei@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>