[5.x] Allow any queued class to be silenced

[lioneaglesolutions]

This PR looks bigger than it is due to moving a chunk of Fixtures and Unit tests into the Feature directory.

What problem does this solve?

This allows any queued class to be silenced - we have a lot of listeners that fire constantly and most of them are trivial so we don't need to see them in the completed list.

Related: #1239

This PR will allow any queued class to be silenced - the initial implementation meant only queued Jobs could be silenced. Now, any Mailable, Notification, Listener, Broadcast or Job can be silenced by implementing the Silenced interface.

The main changes to this PR are;

• moving unit tests to feature directory so the config helper is useable in JobPayload
• inspect the underlying job class to see if it has implemented the Silenced interface
• move the logic to checking if the job is silenced from MarkJobAsComplete into the payload

I wasn't 100% sure on the silenced mailable test but did it the only way I could think to make it easy by adding the mock to the Horizon silenced config