Tests for proxies, covering  #74759.

* bzrlib/transport/http/_urllib2_wrappers.py:
(ProxyHandler.proxy_bypass): Matches against the modified regexp,
not the original domain.

* bzrlib/tests/test_http.py:
(TestProxyHttpServer, TestProxyHttpServer_urllib,
TestProxyHttpServer_pycurl): New classes for proxy tests.

* bzrlib/tests/HttpServer.py:
(HttpServer._http_start): Give access to the port used by the
server socket.
(HttpServer.setUp, HttpServer.tearDown): Handles the 'no_proxy'
env var too.

* bzrlib/tests/HTTPTestUtil.py:
(TestCaseWithTwoWebservers): New class for tests needing two
related web servers.
(FakeProxyRequestHandler): New class to fake a proxy http server.