Extend the doth system test with IPv6 support [GL #2861]

Extend the doth system test with IPv6 support [GL #2861]

This commit ensures that DoH (and DoT) functionality works well via
IPv6 as well.

The changes were made because it turned out that dig could not make
DoH queries against an IPv6 IP address. These tests ensure that such a
bug will not remain unnoticed.

The commit also increases the servers' startup timeout to 25 seconds
because the initial timeout of 14 seconds was too short to generate
(!) eight 4096 bit ephemeral RSA certificates on a heavily loaded CI
runner in some pipeline runs.