matrix tests: tweak import and add __init__.py to ensure tests run.
ADD
zulip/integrations/__init__.py
zulip/integrations/__init__.py
ADD
zulip/integrations/bridge_with_matrix/__init__.py
zulip/integrations/bridge_with_matrix/__init__.py
MODIFY
zulip/integrations/bridge_with_matrix/test_matrix.py
zulip/integrations/bridge_with_matrix/test_matrix.py
-from matrix_bridge import (
+from .matrix_bridge import (
check_zulip_message_validity,
zulip_to_matrix,
)

matrix tests: tweak import and add __init__.py to ensure tests run.