Use Resolver instead of socket.getaddrinfo to decide whether localhost is ipv6.

The results can differ with different resolvers.  Add a CaresResolver
configuration to tox (it was already in travis).

tornado/test/tcpclient_test.py

@@ -67,8 +67,9 @@ class TCPClientTest(AsyncTestCase):
         super(TCPClientTest, self).tearDown()
 
     def skipIfLocalhostV4(self):
-        families = set(sockaddr[0]
-                       for sockaddr in socket.getaddrinfo('localhost', 0))
+        Resolver().resolve('localhost', 0, callback=self.stop)
+        addrinfo = self.wait()
+        families = set(addr[0] for addr in addrinfo)
         if socket.AF_INET6 not in families:
             self.skipTest("localhost does not resolve to ipv6")
 

tox.ini

@@ -31,6 +31,9 @@ envlist =
         py2-twisted, py26-twisted, py2-twistedlayered,
         py3-asyncio, py33-asyncio, py26-trollius, py2-trollius,
 
+        # Alternate Resolvers.
+        py2-caresresolver, py3-caresresolver,
+
         # Other configurations; see comments below.
         py2-monotonic, py3-monotonic,
         py2-opt, py3-opt,
@@ -206,6 +209,17 @@ deps =
      trollius>=0.1.3
 commands = python -m tornado.test.runtests --ioloop=tornado.platform.asyncio.AsyncIOLoop {posargs:}
 
+[testenv:py2-caresresolver]
+basepython = python2.7
+deps = {[testenv:py27-full]deps}
+commands = python -m tornado.test.runtests --resolver=tornado.platform.caresresolver.CaresResolver {posargs:}
+
+[testenv:py3-caresresolver]
+# pycares does not install on python 3.3+
+basepython = python3.2
+deps = {[testenv:py32-full]deps}
+commands = python -m tornado.test.runtests --resolver=tornado.platform.caresresolver.CaresResolver {posargs:}
+
 [testenv:py2-monotonic]
 basepython = python2.7
 deps =