selftests: ublk: add user copy test cases

selftests: ublk: add user copy test cases

The ublk selftests cover every data copy mode except user copy. Add
tests for user copy based on the existing test suite:
- generic_14 ("basic recover function verification (user copy)") based
  on generic_04 and generic_05
- null_03 ("basic IO test with user copy") based on null_01 and null_02
- loop_06 ("write and verify over user copy") based on loop_01 and
  loop_03
- loop_07 ("mkfs & mount & umount with user copy") based on loop_02 and
  loop_04
- stripe_05 ("write and verify test on user copy") based on stripe_03
- stripe_06 ("mkfs & mount & umount on user copy") based on stripe_02
  and stripe_04
- stress_06 ("run IO and remove device (user copy)") based on stress_01
  and stress_03
- stress_07 ("run IO and kill ublk server (user copy)") based on
  stress_02 and stress_04

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