Avoid excluding guards from path building in minimal test networks

choose_good_entry_server() now excludes current entry
guards and their families, unless we're in a test network,
and excluding guards would exclude all nodes.

This typically occurs in incredibly small tor networks,
and those using TestingAuthVoteGuard *

This is an incomplete fix, but is no worse than the previous
behaviour, and only applies to minimal, testing tor networks
(so it's no less secure).

Discovered as part of #13718.