[3.12] gh-109096: Silence test_httpservers fork + threads DeprecationWarning on CGI...

[3.12] gh-109096: Silence test_httpservers fork + threads DeprecationWarning on CGI support (#109471)

[3.12] gh-109096: Silence test_httpservers fork + threads DeprecationWarning on CGI support.

We're not fixing CGIHTTPRequestHandler as it is deprecated in 3.13 to go
away in 3.15.  This just removes noise from our test suite when warnings
are rightfully enabled.

If the long pre-existing fork+threading mix here ever causes anyone
deadlocks as is possible, disabling the test entirely on that platform
makes sense rather than attempting to fix
http.server.CGIHTTPRequestHandler or refactor to not use a threaded
server in the test.