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 1f10186476)
(cherry picked from commit 3dde7c42db)
(cherry picked from commit e787e54ea1)
7394dd8732