This commit extends the 'doth' system test with a set of Strict/Mutual
TLS related checks.
This commit also makes each doth NS instance use its own TLS
certificate that includes FQDN, IPv4, and IPv6 addresses, issued using
a common Certificate Authority, instead of ad-hoc certs.
Extend servers initialisation timeout to 60 seconds to improve the
tests stability in the CI as certain configurations could fail to
initialise on time under load.
