Add tests for all linux pack actions

The contrib linux pack has a few broken actions (check_loadavg, check_processes, dig: #4993, diag_loadavg, netstat: #4947, possibly more) at least on CentOS 8. It's really disheartening to spin up StackStorm and immediately run into broken actions.

We should add tests for those actions to the pack.

The tests from the core pack can be used as a template for writing tests for the linux pack.

I will try to add tests for the dig action in my PR #4993, to serve as a concrete example, but adding tests would be a useful way to introduce somebody to developing StackStorm that would have an immediate impact.