Add mplex tests and fix error in SwarmConn.close

tests/stream_muxer/conftest.py

@@ -0,0 +1,16 @@
+import asyncio
+
+import pytest
+
+from tests.factories import mplex_conn_pair_factory
+
+
+@pytest.fixture
+async def mplex_conn_pair(is_host_secure):
+    mplex_conn_0, swarm_0, mplex_conn_1, swarm_1 = await mplex_conn_pair_factory(
+        is_host_secure
+    )
+    try:
+        yield mplex_conn_0, mplex_conn_1
+    finally:
+        await asyncio.gather(*[swarm_0.close(), swarm_1.close()])

tests/stream_muxer/test_mplex_conn.py

@@ -0,0 +1,6 @@
+import pytest
+
+
+@pytest.mark.asyncio
+async def test_mplex_conn(mplex_conn_pair):
+    conn_0, conn_1 = mplex_conn_pair