tests/net: HTTP proxy client test

With this patch, ProxyTest is implemented and all of the tests in
HttpTest are enabled.

Adding a transparent proxy server implementation proxy.py. Like
testserver.py, this can be provided a socket file descriptor and port
via command-line arguments.

TestServer was refactored to extract ChildProcess and
RandomTCPServerPort, which are now also used by TestProxyServer.

ProxyTest starts TestProxyServer and validates that the request is
sent to the proxy and is routed to the appropriate endpoint of the
downstream server.

The template which adds common tests between HttpTest and HttpsTest
was changed slightly to just take a BThreadedTestCaller<T>&, which
made it simpler to add additional test cases to one suite which are
not appropriate to the other. There wasn't much point in keeping that
template as a member function so I moved it into HttpTest.cpp as a
free function template.

Change-Id: Ied32d6e10bb195d111cae7bbcf0e93168118088b
Reviewed-on: https://review.haiku-os.org/c/haiku/+/2291
Reviewed-by: Adrien Destugues <pulkomandy@gmail.com>