Revive dupsigs system test

Revive dupsigs system test

Correctly source conf.sh in dupsigs test scripts (fix issue introduced
by 093af1c00ac25e4f132fe2442a24e1264aadb28d).

Update dupsigs test for dnssec-dnskey-kskonly default. Since v9.17.20,
the dnssec-dnskey-kskonly is set to yes. Update the test to not expect
the additional RRSIG with ZSK for DNSKEY.

Speed up the test from 20 minutes to 2.5 minutes and make it part of the
default test suite executed in CI.
- decrease number of records to sign from 2000 to 500
- decrease the signing interval by a factor of 6
- shorten the final part of the test after last signing (since nothing
  new happens there)

Finally, clarify misleading comments about (in)sufficient time for zone
re-signing. The time used in the test is in fact sufficient for the
re-signing to happen. If it wasn't, the previous ZSK would end up being
deleted while its signatures would still be present, which is a
situation where duplicate signatures can still happen.