selftests: ublk: don't share backing files between ublk servers

selftests: ublk: don't share backing files between ublk servers

stress_04 is missing a wait between blocks of tests, meaning multiple
ublk servers will be running in parallel using the same backing files.
Add a wait after each section to ensure each backing file is in use by a
single ublk server at a time.

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>