Use the test suite __opts__ instead of custom dict

Better approach now that we're in Salt core.
Move rest_cherrypy mocked methods into individual tests
Added basic smoke-tests for the rest_cherrypy /run entrypoint
Check rest_cherrypy sessions are on before accessing the session lock

Fixes #14205
Merge pull request #14229 from whiteinge/rest_cherrypy-run-sessions

Add rest_cherrypy tests for the /run entrypoint; fix session reference when sessions are disabled