tests: fix warnings in test_network.py

```
electrum/tests/test_network.py::TestNetwork::test_can_connect_during_backward
electrum/tests/test_network.py::TestNetwork::test_chain_false_during_binary
electrum/tests/test_network.py::TestNetwork::test_fork_conflict
electrum/tests/test_network.py::TestNetwork::test_fork_noconflict
  /tmp/cirrus-ci-build/electrum/interface.py:410: RuntimeWarning: coroutine 'Interface.run' was never awaited
    task = await self.network.taskgroup.spawn(self.run())
  Enable tracemalloc to get traceback where the object was allocated.
  See https://docs.pytest.org/en/stable/how-to/capture-warnings.html#resource-warnings for more info.
```

closes https://github.com/spesmilo/electrum/pull/7817

electrum/tests/test_network.py

@@ -7,19 +7,18 @@ from electrum.simple_config import SimpleConfig
 from electrum import blockchain
 from electrum.interface import Interface, ServerAddr
 from electrum.crypto import sha256
+from electrum.util import OldTaskGroup
 from electrum import util
 
 from . import ElectrumTestCase
 
 
-class MockTaskGroup:
-    async def spawn(self, x): return
-
 class MockNetwork:
-    taskgroup = MockTaskGroup()
 
     def __init__(self):
         self.asyncio_loop = util.get_asyncio_loop()
+        self.taskgroup = OldTaskGroup()
+
 
 class MockInterface(Interface):
     def __init__(self, config):
@@ -32,6 +31,7 @@ class MockInterface(Interface):
                                                 parent=None, forkpoint_hash=constants.net.GENESIS, prev_hash=None)
         self.tip = 12
         self.blockchain._size = self.tip + 1
+
     async def get_block_header(self, height, assert_mode):
         assert self.q.qsize() > 0, (height, assert_mode)
         item = await self.q.get()
@@ -40,6 +40,10 @@ class MockInterface(Interface):
         assert assert_mode in item['mock'], (assert_mode, item)
         return item
 
+    async def run(self):
+        return
+
+
 class TestNetwork(ElectrumTestCase):
 
     @classmethod