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.

lib/dns/tests/testdata/master/master18.data

@@ -0,0 +1,10 @@
+$TTL 1000
+@		in	soa	localhost. postmaster.localhost. (
+				1993050801	;serial
+				3600		;refresh
+				1800		;retry
+				604800		;expiration
+				3600 )		;minimum
+
+$INCLUDE "testkeys/Kexample.+008+20386.key";
+$INCLUDE "testkeys/Kexample.+008+37464.key";