tests: toy_server: move start/stop logic

diff --git a/tests/test_interface.py b/tests/test_interface.py
index 6087a00..d1dccd1 100644
--- a/tests/test_interface.py
+++ b/tests/test_interface.py
@@ -110,21 +110,17 @@ class TestInterface(ElectrumTestCase):
     async def asyncSetUp(self):
         await super().asyncSetUp()
         self._toyserver = ToyServer()
-        session_factory = partial(ToyServerSession, toyserver=self._toyserver)
-        self._server: asyncio.base_events.Server = await aiorpcx.serve_rs(session_factory, "127.0.0.1")
-        server_socket_addr = self._server.sockets[0].getsockname()
-        self._server_port = server_socket_addr[1]
+        await self._toyserver.start()
         self.network = MockNetwork(config=self.config)
 
     async def asyncTearDown(self):
         if self.network.interface:
             await self.network.interface.close()
-        self._server.close()
-        await self._server.wait_closed()
+        await self._toyserver.stop()
         await super().asyncTearDown()
 
     async def _start_iface_and_wait_for_sync(self):
-        interface = Interface(network=self.network, server=ServerAddr(host="127.0.0.1", port=self._server_port, protocol="t"))
+        interface = Interface(network=self.network, server=ServerAddr(host="127.0.0.1", port=self._toyserver.server_port, protocol="t"))
         interface.client_name = lambda: "alice"
         self.network.interface = interface
         async with util.async_timeout(5):
diff --git a/tests/toyserver.py b/tests/toyserver.py
index 7dfce4a..e19be22 100644
--- a/tests/toyserver.py
+++ b/tests/toyserver.py
@@ -2,7 +2,9 @@
 # Distributed under the MIT software license, see the accompanying
 # file LICENCE or http://www.opensource.org/licenses/mit-license.php
 
+import asyncio
 import collections
+from functools import partial
 from typing import Optional, Sequence, Iterable, List, Set
 
 import aiorpcx
@@ -32,6 +34,9 @@ _BLOCK_HEADERS: List[bytes] = [
 class ToyServer:
     """Electrum Server backend"""
 
+    asyncio_server: asyncio.base_events.Server
+    server_port: int
+
     def __init__(self):
         self.sessions = set()  # type: Set[ToyServerSession]
         self._block_headers = _BLOCK_HEADERS[:]
@@ -40,6 +45,16 @@ class ToyServer:
         self.sh_to_spending_txids = collections.defaultdict(set)  # type: dict[str, set[str]]
         self.txs = {}  # type: dict[str, bytes]
 
+    async def start(self):
+        session_factory = partial(ToyServerSession, toyserver=self)
+        self.asyncio_server = await aiorpcx.serve_rs(session_factory, "127.0.0.1")
+        server_socket_addr = self.asyncio_server.sockets[0].getsockname()
+        self.server_port = server_socket_addr[1]
+
+    async def stop(self):
+        self.asyncio_server.close()
+        await self.asyncio_server.wait_closed()
+
     def get_session_by_name(self, client_name: str) -> 'ToyServerSession':
         found_sessions = [
             session for session in self.sessions