Fix flakiness in the doth reconfig test

Due to the fact that the primary nameserver creates a lot of TLS
contexts, its reconfiguration could take too much time on the CI,
leading to spurious test failures, while in reality it works just
fine.

This commit adds a separate instance for this test which does not use
ephemeral keys (these are costly to generate) and creates minimal
amount of TLS contexts.

bin/tests/system/doth/setup.sh

@@ -27,3 +27,4 @@ done
 copy_setports ns1/named.conf.in ns1/named.conf
 copy_setports ns2/named.conf.in ns2/named.conf
 copy_setports ns3/named.conf.in ns3/named.conf
+copy_setports ns4/named.conf.in ns4/named.conf