Add pycares to the -full tox configs and remove custom resolver tests.

The resolvers are tested in netutil_test when their dependencies are
available, and re-running the entire test suite with a different resolver
is unlikely to expose any new problems.  (the resolver tests are still
present in travis.yml for now, though)

tox.ini

@@ -30,11 +30,6 @@ envlist =
         py2-twisted, py26-twisted, py2-twistedlayered,
         py33-asyncio, py2-trollius,
 
-        # Alternate resolvers.
-        py2-threadedresolver, py3-threadedresolver,
-        py2-twistedresolver,
-        py2-caresresolver, py32-caresresolver,
-
         # Other configurations; see comments below.
         py2-monotonic, py3-monotonic,
         py2-opt, py3-opt,
@@ -68,6 +63,7 @@ deps =
      {[testenv:py26]deps}
      futures
      mock
+     pycares
      pycurl
      twisted>=11.0.0
 
@@ -77,6 +73,7 @@ setenv = TORNADO_EXTENSION=1
 deps =
      futures
      mock
+     pycares
      pycurl
      twisted>=11.0.0
 
@@ -95,21 +92,24 @@ basepython = python3.2
 setenv = TORNADO_EXTENSION=1
 deps =
      mock
+     pycares
      pycurl>=7.19.3
 
 [testenv:py33-full]
 basepython = python3.3
 setenv = TORNADO_EXTENSION=1
+# pycares installation currently fails on py33
+# (https://github.com/pypa/pip/pull/816)
 deps =
      pycurl>=7.19.3
 
 [testenv:pypy-full]
 # This configuration works with pypy 1.9.  pycurl installs ok but
-# curl_httpclient doesn't work.  Twisted works most of the time, but
-# its tests seem flaky under pypy, so we don't run it for now.  Also
-# note that travis-ci does not yet have support for building C
-# extensions with pypy, so we don't try to run this configuration
-# there.
+# curl_httpclient doesn't work.  pycares also installs but doesn't
+# work.  Twisted works most of the time, but its tests seem flaky
+# under pypy, so we don't run it for now.  Also note that travis-ci
+# does not yet have support for building C extensions with pypy, so we
+# don't try to run this configuration there.
 basepython = pypy
 deps =
      futures
@@ -176,37 +176,6 @@ deps =
      twisted
 commands = python -m tornado.test.runtests --ioloop=tornado.platform.asyncio.AsyncIOLoop {posargs:}
 
-[testenv:py2-threadedresolver]
-basepython = python2.7
-deps = {[testenv:py27-full]deps}
-commands = python -m tornado.test.runtests --resolver=tornado.netutil.ThreadedResolver {posargs:}
-
-[testenv:py3-threadedresolver]
-basepython = python3.3
-deps = {[testenv:py33-full]deps}
-commands = python -m tornado.test.runtests --resolver=tornado.netutil.ThreadedResolver {posargs:}
-
-[testenv:py2-twistedresolver]
-basepython = python2.7
-deps = {[testenv:py27-full]deps}
-commands = python -m tornado.test.runtests --resolver=tornado.platform.twisted.TwistedResolver {posargs:}
-
-[testenv:py2-caresresolver]
-basepython = python2.7
-deps =
-     {[testenv:py27-full]deps}
-     pycares
-commands = python -m tornado.test.runtests --resolver=tornado.platform.caresresolver.CaresResolver {posargs:}
-
-# pycares installation currently fails on py33
-# (https://github.com/pypa/pip/pull/816)
-[testenv:py32-caresresolver]
-basepython = python3.2
-deps =
-     {[testenv:py32-full]deps}
-     pycares
-commands = python -m tornado.test.runtests --resolver=tornado.platform.caresresolver.CaresResolver {posargs:}
-
 [testenv:py2-monotonic]
 basepython = python2.7
 deps =