Workaround for pytest issue #3542 (#346)

Implement workaround for pytest issue [#3542](pytest-dev/pytest#3542)

There seems to be a bug in pytest where metafunc.parametrize breaks
fixture scope. By creating a fixture and parameterizing it indirectly
scope can be maintained

testinfra/plugin.py

@@ -66,6 +66,11 @@ def TestinfraBackend(host):
     return host.backend
 
 
+@pytest.fixture(scope="module")
+def _testinfra_host(request):
+    return request.param
+
+
 @pytest.fixture(scope="module")
 def host(_testinfra_host):
     return _testinfra_host
@@ -149,7 +154,7 @@ def pytest_generate_tests(metafunc):
         params = sorted(params, key=lambda x: x.backend.get_pytest_id())
         ids = [e.backend.get_pytest_id() for e in params]
         metafunc.parametrize(
-            "_testinfra_host", params, ids=ids, scope="module")
+            "_testinfra_host", params, ids=ids, scope="module", indirect=True)
 
 
 def pytest_collection_finish(session):