Add test for SIG in prequisites of dynamic update

Add test for SIG in prequisites of dynamic update

Make sure the nameserver correctly handles SIG records in the
prerequisites of the dynamic update. The first check is to ensure that
the prerequisites are not examined prior to checking the credentials.

The second test case checks that the SIG present prerequisite is
examined and therefore refuses the update. Also this should not trigger
an assertion failure in dns__db_findrdataset() (due to the REQUIRE()
only accepted dns_rdatatype_rrsig when the covers parameter was set).

(cherry picked from commit 51f27fda46870acea69835f82835ca857c000bd0)