Tests DnsStream event handling, both for plain TCP DNS and DNS over TLS.
The DoT test requires the "openssl s_server" command line tool to mock a simple
TLS server. Thus the test's TLS part is skipped if openssl it not available.
The test works for both DNS_OVER_TLS_USE_GNUTLS and DNS_OVER_TLS_USE_OPENSSL.
The DoT case fails due to a bug, which is fixed on the next commit.
