test: add __init__.py to plugins test directories to make them Python packages

The test/integration/plugins directory and its subdirectories were
missing __init__.py files, preventing Python from treating them as
packages. This caused local/relative imports to fail when running
plugin tests in a deployed Marvin environment, making it impossible
to modularize tests for plugins.

Fixes #3679

Author: DaanHoogland

test/integration/plugins/__init__.py

@@ -0,0 +1,16 @@
+# Licensed to the Apache Software Foundation (ASF) under one
+# or more contributor license agreements.  See the NOTICE file
+# distributed with this work for additional information
+# regarding copyright ownership.  The ASF licenses this file
+# to you under the Apache License, Version 2.0 (the
+# "License"); you may not use this file except in compliance
+# with the License.  You may obtain a copy of the License at
+#
+#   http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing,
+# software distributed under the License is distributed on an
+# "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+# KIND, either express or implied.  See the License for the
+# specific language governing permissions and limitations
+# under the License.