tests: ensure cleanup with configs via get_config()

Also done in test_pluginmanager, although no resource warnings are
there at least.

Fixes https://github.com/pytest-dev/pytest/issues/4355.

src/_pytest/pytester.py

@@ -375,6 +375,15 @@ def testdir(request, tmpdir_factory):
     return Testdir(request, tmpdir_factory)
 
 
+@pytest.fixture
+def _config_for_test():
+    from _pytest.config import get_config
+
+    config = get_config()
+    yield config
+    config._ensure_unconfigure()  # cleanup, e.g. capman closing tmpfiles.
+
+
 rex_outcome = re.compile(r"(\d+) ([\w-]+)")