test: ensure all returned interface versions are 1

Even where our client pretends to have higher interface versions, we
always send back version 1 from the EIS implementation.

This test probably won't really trigger anything useful until we switch to
version 2 somewhere, so let's hope the code works...

test/test_protocol.py

@@ -160,13 +160,16 @@ class Ei:
         assert isinstance(setup, EiHandshake)
         return setup
 
-    def init_default_sender_connection(self) -> None:
+    def init_default_sender_connection(
+        self, interface_versions: dict[str, int] = {}
+    ) -> None:
         setup = self.handshake
         self.send(setup.HandshakeVersion(VERSION_V(1)))
         self.send(setup.ContextType(EiHandshake.EiContextType.SENDER))
         self.send(setup.Name("test client"))
         for iname in filter(lambda i: i != InterfaceName.EI_HANDSHAKE, InterfaceName):
-            self.send(setup.InterfaceVersion(iname, VERSION_V(1)))
+            version = interface_versions.get(iname, VERSION_V(1))
+            self.send(setup.InterfaceVersion(iname, version))
         self.send(setup.Finish())
         self.dispatch()
 
@@ -406,7 +409,42 @@ class TestEiProtocol:
             if call.name == "InterfaceVersion":
                 announced_interfaces.append((call.args["name"], call.args["version"]))
 
-        assert (InterfaceName.EI_CALLBACK, 1) in announced_interfaces
+        assert (InterfaceName.EI_CALLBACK, VERSION_V(1)) in announced_interfaces
+
+        # Right now all our versions are 1, so let's ensure that's true
+        for _, version in announced_interfaces:
+            assert version == VERSION_V(1)
+
+        eis.terminate()
+
+    def test_server_sends_min_interface_version(self, eis):
+        ei = eis.ei
+        ei.dispatch()
+
+        setup = ei.handshake
+
+        # Assign a random high number of the interfaces we claim to support
+        interface_versions: dict[str, int] = {
+            iface.name: idx + 3
+            for idx, iface in enumerate(InterfaceName)
+            if iface != InterfaceName.EI_HANDSHAKE
+        }
+
+        ei.init_default_sender_connection(interface_versions=interface_versions)
+        ei.dispatch()
+        ei.wait_for_connection()
+
+        announced_interfaces = []
+
+        for call in setup.calllog:
+            if call.name == "InterfaceVersion":
+                announced_interfaces.append((call.args["name"], call.args["version"]))
+
+        assert (InterfaceName.EI_CALLBACK, VERSION_V(1)) in announced_interfaces
+
+        # Right now all our EIS versions are 1, despite whatever we announce
+        for _, version in announced_interfaces:
+            assert version == VERSION_V(1)
 
         eis.terminate()