Ensure pylint runs in some tests paths

Description

We used to lack a __init__.py file in various tests directories, which meant pylint would not run

Changes proposed in this merge request:

• Adding a __init__.py file in tests/plugins
• Adding a __init__.py file in tests/remoteexecution
• Adding a __init__.py file in tests/sandboxes
• Fixing all pylint errors there

Disabling the redefined-outer-name must be done per file, as Pylint doesn't support per directory configuration.

See https://github.com/PyCQA/pylint/issues/618 for the upstream issue