tests: add aux delayzero_dev support

tests: add aux delayzero_dev  support

Just like we have 'writeerror_dev' supporting creation of device
which 'readable' segment and segments where write will fail we
have now support for delay zero mappings.

This is useful if we want to 'fake' large writing areas where we do
not really care about the actual 'disk' content - since we test
operation logic and it doesn't matter we read and write zeroes.
With combination with 'delay' target we can create specific mappings
and avoid using large memory areas of ramdisk.