Fix key ID extraction in the "dnssec" system test

Fix key ID extraction in the "dnssec" system test

Simply looking for the key ID surrounded by spaces in the tested
dnssec-signzone output file is not a precise enough method of checking
for signatures prepared using a given key ID: it can be tripped up by
cross-algorithm key ID collisions and certain low key IDs (e.g. 60, the
TTL specified in bin/tests/system/dnssec/signer/example.db.in), which
triggers false positives for the "dnssec" system test.  Make key ID
extraction precise by using an awk script which operates on specific
fields.

(cherry picked from commit a40c60e4c1d974a0d9dbad513a1a78fa2a4025b5)