test: Add OVS internal interface MTU modification test

Add two tier1 test case:
    * `test_create_internal_port_with_explict_mtu`
    * `test_change_mtu_of_existing_internal_port`

Signed-off-by: Gris Ge <fge@redhat.com>

tests/integration/ovs_test.py

@@ -436,3 +436,38 @@ class TestOvsPatch:
         assertlib.assert_absent(BRIDGE0)
         assertlib.assert_absent(PATCH0)
         assertlib.assert_absent(PATCH1)
+
+
+@pytest.mark.tier1
+def test_create_internal_port_with_explict_mtu():
+    bridge = Bridge(BRIDGE1)
+    bridge.add_internal_port(
+        PORT1,
+        mac=MAC1,
+        mtu=1501,
+        ipv4_state={
+            InterfaceIPv4.ENABLED: True,
+            InterfaceIPv4.ADDRESS: [
+                {
+                    InterfaceIPv4.ADDRESS_IP: "192.0.2.1",
+                    InterfaceIPv4.ADDRESS_PREFIX_LENGTH: 24,
+                }
+            ],
+        },
+    )
+
+    with bridge.create() as state:
+        assertlib.assert_state_match(state)
+
+    assertlib.assert_absent(BRIDGE1)
+    assertlib.assert_absent(PORT1)
+
+
+@pytest.mark.tier1
+def test_change_mtu_of_existing_internal_port(bridge_with_ports):
+    state = statelib.show_only((PORT1,))
+    state[Interface.KEY][0][Interface.MTU] = 1501
+
+    libnmstate.apply(state)
+
+    assertlib.assert_state_match(state)

tests/integration/testlib/ovslib.py

@@ -69,7 +69,14 @@ class Bridge:
             ] = mode
 
     def add_internal_port(
-        self, name, *, mac=None, ipv4_state=None, ovs_db=None, patch_state=None
+        self,
+        name,
+        *,
+        mac=None,
+        ipv4_state=None,
+        ovs_db=None,
+        patch_state=None,
+        mtu=None,
     ):
         ifstate = {
             Interface.NAME: name,
@@ -77,6 +84,8 @@ class Bridge:
         }
         if mac:
             ifstate[Interface.MAC] = mac
+        if mtu:
+            ifstate[Interface.MTU] = mtu
         if ipv4_state:
             ifstate[Interface.IPV4] = ipv4_state
         if ovs_db: