dnsdist: Fix the unit tests to handle v4-only or v6-only connectivity

[rgacogne]

Short description

In the load-balancing policies tests, we simply do not try to open a connection to the backend. In the TCP and HTTP/2 tests, we detect which protocol is available by trying to connect a socket, and use v6 if it is available and v4 otherwise.

Closes #10403.

Checklist

I have:

• read the CONTRIBUTING.md document
• compiled this code
• tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)