Refactor interop tests and factories

- Add `close` and `disconnect` in `Host`
- Add `close` and `close_peer` in `Network`
- Change `IListener.close` to async, to await for server's closing
- Add factories for security transports, and modify `HostFactory`

tests/conftest.py

@@ -0,0 +1,33 @@
+import asyncio
+
+import pytest
+
+from .configs import LISTEN_MADDR
+from .factories import HostFactory
+
+
+@pytest.fixture
+def is_host_secure():
+    return False
+
+
+@pytest.fixture
+def num_hosts():
+    return 3
+
+
+@pytest.fixture
+async def hosts(num_hosts, is_host_secure):
+    _hosts = HostFactory.create_batch(num_hosts, is_secure=is_host_secure)
+    await asyncio.gather(
+        *[_host.get_network().listen(LISTEN_MADDR) for _host in _hosts]
+    )
+    try:
+        yield _hosts
+    finally:
+        # TODO: It's possible that `close` raises exceptions currently,
+        #   due to the connection reset things. Though we are not so careful about that when
+        #   cleaning up the tasks, it is probably better to handle the exceptions properly.
+        await asyncio.gather(
+            *[_host.close() for _host in _hosts], return_exceptions=True
+        )