pytest: make test_plugin_slowinit a test again

tests/plugins/slow_init.py

@@ -8,7 +8,7 @@ plugin = Plugin()
 @plugin.init()
 def init(options, configuration, plugin):
     plugin.log("slow_init.py initializing {}".format(configuration))
-    time.sleep(2)
+    time.sleep(11)
 
 
 plugin.run()

tests/test_plugin.py

@@ -97,6 +97,7 @@ def test_plugin_slowinit(node_factory):
     """Tests that the 'plugin' RPC command times out if plugin doesnt respond"""
     n = node_factory.get_node()
 
+    with pytest.raises(RpcError, match="Timed out while waiting for plugin response"):
         n.rpc.plugin_start(os.path.join(os.getcwd(), "tests/plugins/slow_init.py"))
 
     # It's not actually configured yet, see what happens;