pytest plugins _cannot_ be installed by a non-top-level conftest file.
Now that provider tests are moved to a nested directory we need to plumb
the plugin in a different way.

The plugin is now provided on the command line for breeze testing
commands.

The config files which contain exceptions for forbidden_warnings were
set in pytest config in the conftest files. The plugin is now loaded at
the top-level and before conftest files are. Move the config into the
python module for the plugin itself. There should be no harm in
configuring all exceptions for each test type