selftests: ublk: refactor test_loop_08 into separate functions

selftests: ublk: refactor test_loop_08 into separate functions

Encapsulate each test case in its own function for better organization
and maintainability:

- _setup_device(): device and backfile initialization
- _test_fill_and_verify(): initial data population
- _test_corrupted_reftag(): reftag corruption detection test
- _test_corrupted_data(): data corruption detection test
- _test_bad_apptag(): apptag mismatch detection test

Also fix temp file creation to use ${UBLK_TEST_DIR}/fio_err_XXXXX instead of
creating in current directory.

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