testing: fix warning issued by test_cache_writefail_cachfile_silent

Remove this message which was shown in the warning summary of pytest's
own testsuite:

    testing/test_cacheprovider.py::TestNewAPI::test_cache_writefail_cachfile_silent
      testing/test_cacheprovider.py:40: PytestCacheWarning: could not create cache path /tmp/pytest-of-ran/pytest-2/test_cache_writefail_cachfile_silent0/.pytest_cache/v/test/broken
        cache.set("test/broken", [])

Author: bluetech

testing/test_cacheprovider.py

@@ -31,7 +31,7 @@ def test_config_cache_dataerror(self, testdir):
         val = config.cache.get("key/name", -2)
         assert val == -2
 
-    @pytest.mark.filterwarnings("default")
+    @pytest.mark.filterwarnings("ignore:could not create cache path")
     def test_cache_writefail_cachfile_silent(self, testdir):
         testdir.makeini("[pytest]")
         testdir.tmpdir.join(".pytest_cache").write("gone wrong")