Add a framework for dns__zone_updatesigs() unit tests

Add a framework for dns__zone_updatesigs() unit tests

Add a new ATF test, sigs_test, containing everything required to start
defining test cases for dns__zone_updatesigs().  The framework is
written in a way which ensures that changes to zone database applied by
any dns__zone_updatesigs() invocation are preserved between subsequent
checks.

(cherry picked from commit 1f10186476042562f2858e14331b5e76b2e54b83)
(cherry picked from commit 3dde7c42db917d4c27e0abba9c18dc607bde4502)
(cherry picked from commit e787e54ea18c0f611af97ea2066775d7daadb435)