test: Retry on assert_absent()

There is a race when we delete OVS interface via NM and querying OVS
interface right after it. NM might still how this OVS interface in its
cache.

Instead of instant fail, we retry in `assert_absent()`.

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

tests/integration/testlib/assertlib.py

@@ -47,9 +47,14 @@ def assert_state(desired_state_data):
 
 def assert_absent(*ifnames):
     """Assert that a interface is not present in the current state"""
-
-    current_state = statelib.show_only(ifnames)
-    assert not current_state[Interface.KEY]
+    for i in range(0, RETRY_COUNT):
+        current_state = statelib.show_only(ifnames)
+        if not current_state[Interface.KEY]:
+            return
+        elif i == RETRY_COUNT - 1:
+            assert not current_state[Interface.KEY]
+        else:
+            time.sleep(0.5)
 
 
 def assert_state_match(desired_state_data):