create the previously missing cache fixture

there was a disabled test

_pytest/cacheprovider.py

@@ -171,6 +171,10 @@ def pytest_configure(config):
     config.pluginmanager.register(LFPlugin(config), "lfplugin")
 
 
+@pytest.fixture
+def cache(request):
+    return request.config.cache
+
 def pytest_report_header(config):
     if config.option.verbose:
         relpath = py.path.local().bestrelpath(config.cache._cachedir)

testing/test_cache.py

@@ -36,7 +36,7 @@ class TestNewAPI:
         assert result.ret == 0
         result.stdout.fnmatch_lines(["*1 passed*"])
 
-    def XXX_test_cachefuncarg(self, testdir):
+    def test_cachefuncarg(self, testdir):
         testdir.makepyfile("""
             import pytest
             def test_cachefuncarg(cache):