bridge_with_matrix: Fix test.
It was broken by commit
MODIFY
zulip/integrations/bridge_with_matrix/test_matrix.py
zulip/integrations/bridge_with_matrix/test_matrix.py
@@ -18,7 +18,7 @@ sample_config_path = "matrix_test.conf"
sample_config_text = """[matrix]
host = https://matrix.org
-username = @username@matrix.org
+username = @username:matrix.org
password = password
room_id = #zulip:matrix.org

bridge_with_matrix: Fix test.

It was broken by commit e876a0541e68041d8f3f098b30565d6c3dab455a
(#701).

Signed-off-by: Anders Kaseorg <anders@zulip.com>