Write unit tests for channel_matches_target_addr_for_extend()

In legacy/trac#33817 (moved), we modify channel_matches_target_addr_for_extend() to take an IPv4 and an IPv6 address.

We also modify channel_get_for_extend() to take an IPv4 and an IPv6 address. It has existing tests, but they ignore IP addresses. (And mock the matches_target() method, which is called by channel_matches_target_addr_for_extend().)

It would be useful to have tests that check that a channel matches, if its IP address matches the IPv4 or IPv6 address. But it's not urgent. (And the actual changes to the function are pretty trivial.)

The setup for these tasks is a bit tricky, we need to set up a channel_tls_t and an associated connection_t with a real_addr. (Note that legacy/trac#33898 (moved) may change the name of real_addr.)

This task is not urgent or important. Anyone can do this task.