Add UTs for diags code
Add UTs for diags code

UTs for the newly added _log_interfaces diags helper function.

caseydavenport/calico-kubernetes,alexhersh/calico-kubernetes,alexhersh/calico-kubernetes,Symmetric/calico-kubernetes,caseydavenport/calico-kubernetes,projectcalico/calico-kubernetes,projectcalico/calico-kubernetes,Symmetric/calico-kubernetes