This uses c-ares under the hood and supports the CURL_DNS_SERVER
environment variable - for debug builds only. The getaddrinfo()
replacement function is only used if CURL_DNS_SERVER is set to make a
debug build work more like a release version without the variable set.
'override-dns' is a new feature for the test suite when curl can be told
to use a dedicated DNS server, and test 2102 is the first to require
this.
Requires c-ares 1.26.0 or later.
Closes#17134
Currently the DNS server only responds to A and AAAA queries. It always
responds with a fixed response: the localhost address. Three times.
It should work fine over either IPv4 or IPv6, but I don't think it
matters much for curl testing.
The idea is to allow curl tests to use "normal" DNS hostnames (using the
normal name resolving code paths) and still use the local test servers.
This setup currently only works if curl is built with c-ares because
redirecting DNS requests to our test server when using getaddrinfo() is
not easy.
This should be extended to respond to HTTPS queries as well to allow
more testing there, as c-ares is always used for that.
Test 2102 is the first test using this.
Closes#17015
