Add a helper function to facilitate preparing dns_diff_t structures

Add a helper function to facilitate preparing dns_diff_t structures

Implement dns_test_difffromchanges(), a function which enables preparing
a dns_diff_t structure from a mostly-textual representation of zone
database changes to be applied.  This will improve readability of test
case definitions by allowing contents of a dns_diff_t structure, passed
e.g. to update_sigs(), to be represented in a human-friendly manner.

(cherry picked from commit 3c22af0d35ada9f83557845d481ef1c9257ae383)
(cherry picked from commit d4c603eb8ac7c663d3fb5063259067d9f4600968)
(cherry picked from commit 1f454b8d91bff9ecb70307557d1a30e24cd6dc44)