Add unit test for graphviz

[luolanzone]

Part of #4142 (Improve unit test coverage)
Signed-off-by: Lan Luo luola@vmware.com

[codecov]

Codecov Report

Merging #4263 (63b2f7f) into main (e7486d5) will increase coverage by 0.55%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #4263      +/-   ##
==========================================
+ Coverage   62.24%   62.79%   +0.55%     
==========================================
  Files         385      385              
  Lines       54513    54513              
==========================================
+ Hits        33929    34232     +303     
+ Misses      18105    17745     -360     
- Partials     2479     2536      +57     

Flag	Coverage Δ
integration-tests	35.00% <ø> (-0.14%)	⬇️
kind-e2e-tests	48.99% <ø> (+0.84%)	⬆️
unit-tests	44.23% <ø> (+0.39%)	⬆️

Impacted Files	Coverage Δ
pkg/agent/cniserver/ipam/ipam_service.go	76.40% <0.00%> (-12.36%)	⬇️
...ntrollers/multicluster/serviceexport_controller.go	76.62% <0.00%> (-2.86%)	⬇️
...catesigningrequest/ipsec_csr_signing_controller.go	61.65% <0.00%> (-2.46%)	⬇️
...trollers/multicluster/resourceexport_controller.go	77.86% <0.00%> (-2.19%)	⬇️
pkg/agent/openflow/multicluster.go	44.85% <0.00%> (-1.87%)	⬇️
...lticluster/commonarea/resourceimport_controller.go	79.18% <0.00%> (-1.86%)	⬇️
pkg/agent/proxy/proxier.go	69.13% <0.00%> (-1.55%)	⬇️
pkg/controller/ipam/antrea_ipam_controller.go	75.25% <0.00%> (-1.01%)	⬇️
pkg/agent/util/net.go	53.55% <0.00%> (-0.75%)	⬇️
pkg/ovs/openflow/ofctrl_bridge.go	77.26% <0.00%> (-0.58%)	⬇️
... and 22 more

[tnqn]

LGTM

[tnqn]

/skip-all