RelayServer: use in-memory DB for tests

This avoids problems where a leftover DB from one run causes "wrong
code" errors in the next.

src/wormhole/servers/cmd_server.py

@@ -10,7 +10,8 @@ class MyPlugin:
         # accept --reactor= selection
         from .relay import RelayServer
         return RelayServer(self.args.rendezvous, self.args.transit,
-                           self.args.advertise_version)
+                           self.args.advertise_version,
+                           "relay.sqlite")
 
 def start_server(args):
     from twisted.python import usage

src/wormhole/servers/relay.py

@@ -383,9 +383,10 @@ class Root(resource.Resource):
         self.putChild("", static.Data("Wormhole Relay\n", "text/plain"))
 
 class RelayServer(service.MultiService):
-    def __init__(self, relayport, transitport, advertise_version):
+    def __init__(self, relayport, transitport, advertise_version,
+                 db_url=":memory:"):
         service.MultiService.__init__(self)
-        self.db = get_db("relay.sqlite")
+        self.db = get_db(db_url)
         welcome = {
             "current_version": __version__,
             # adding .motd will cause all clients to display the message,