Tune the performance of oldsigs test

Tune the performance of oldsigs test

The oldsigs test was checking only for the validity of the A
a.oldsigs.example. resource record and associated DNSSEC signature while
the zone might not have been fully signed yet leading to validation
failures because of bogus signatures on the validation path.

This commit changes the test to test that all old signatures in the
oldsigs.example. zone were replaced and the zone is fully resigned
before running the main check.