What are best practices for running Pipeline tests into a CI framework?
Unit testing in an alternate CD/CI platform would be overkill in every situation I've built one.  The shared-pipeline code repo you connect to Jenkins will be triggered by a Jenkinsfile.  I suggest you define any and all testing in that particular Jenkinsfile - and keep this one simple enough that extensive testing of the Jenkinsfile defining tests for your Jenkinsfile does not also need a Jenkinsfile defining tests.