selftests: ublk: replace assert() with ublk_assert()

selftests: ublk: replace assert() with ublk_assert()

Replace assert() with ublk_assert() since it is often triggered in daemon,
and we may get nothing shown in terminal.

Add ublk_assert(), so we can log something to syslog when assert() is
triggered.

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