Enable skipping tests based on --reactor.

conftest.py

@@ -35,6 +35,14 @@ def pytest_collection_modifyitems(session, config, items):
     except ImportError:
         pass
 
+
 @pytest.fixture()
 def reactor_pytest(request):
     request.cls.reactor_pytest = request.config.getoption("--reactor")
+    return request.cls.reactor_pytest
+
+
+@pytest.fixture(autouse=True)
+def only_asyncio(request, reactor_pytest):
+    if request.node.get_closest_marker('only_asyncio') and reactor_pytest != 'asyncio':
+        pytest.skip('This test is only run with --reactor-asyncio')

pytest.ini

@@ -19,6 +19,8 @@ addopts =
     --ignore=docs/topics/telnetconsole.rst
     --ignore=docs/utils
 twisted = 1
+markers =
+    only_asyncio: marks tests as only enabled when --reactor=asyncio is passed
 flake8-ignore =
     # Files that are only meant to provide top-level imports are expected not
     # to use any of their imports: