Limit anyio tests to installed backends

src/tests/conftest.py

@@ -1,7 +1,15 @@
+import importlib.util
 import logging
 import pytest
 
 
+@pytest.fixture(
+    params=["asyncio"] + (["trio"] if importlib.util.find_spec("trio") else [])
+)
+def anyio_backend(request):
+    return request.param
+
+
 @pytest.fixture(autouse=True)
 def mute_logging():
     logging.getLogger().setLevel(logging.WARNING)