Test: use mock server in addon tests

[charlie0129]

Signed-off-by: Charlie Chiang charlie_c_0129@outlook.com

Description of your changes

The addon helper tests previously used real upstream KubeVela repository, which will sometimes change and make this test flacky.

This PR sets up a local mock server instead, in an effort to make this test more stable.

Fixes #

I have:

• Read and followed KubeVela's contribution process.
• Related Docs updated properly. In a new feature or configuration option, an update to the documentation is necessary.
• Run make reviewable to ensure this PR is ready for review.
• Added backport release-x.y labels to auto-backport this PR if necessary.

How has this code been tested

Special notes for your reviewer

[codecov]

Codecov Report

Base: 61.15% // Head: 61.11% // Decreases project coverage by -0.04% ⚠️

Coverage data is based on head (44eaa1d) compared to base (5d6848b).
Patch has no changes to coverable lines.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5286      +/-   ##
==========================================
- Coverage   61.15%   61.11%   -0.05%     
==========================================
  Files         308      308              
  Lines       46562    46562              
==========================================
- Hits        28476    28456      -20     
- Misses      15126    15160      +34     
+ Partials     2960     2946      -14     

Flag	Coverage Δ
apiserver-e2etests	32.42% <ø> (-0.06%)	⬇️
apiserver-unittests	36.26% <ø> (+0.05%)	⬆️
core-unittests	55.21% <ø> (+0.04%)	⬆️
e2e-multicluster-test	19.02% <ø> (-0.04%)	⬇️
e2e-rollout-tests	20.72% <ø> (-0.01%)	⬇️
e2etests	26.31% <ø> (-0.04%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
pkg/oam/util/helper.go	61.91% <0.00%> (-2.09%)	⬇️
...troller/core.oam.dev/v1alpha2/application/apply.go	86.07% <0.00%> (-1.90%)	⬇️
pkg/apiserver/event/sync/store.go	69.68% <0.00%> (-1.81%)	⬇️
pkg/cue/definition/template.go	66.46% <0.00%> (-1.50%)	⬇️
pkg/apiserver/domain/service/workflow.go	52.80% <0.00%> (-1.35%)	⬇️
pkg/multicluster/virtual_cluster.go	76.19% <0.00%> (-1.10%)	⬇️
...ller/core.oam.dev/v1alpha2/application/revision.go	71.22% <0.00%> (-0.91%)	⬇️
...ler/core.oam.dev/v1alpha2/application/generator.go	86.37% <0.00%> (-0.82%)	⬇️
pkg/addon/addon.go	64.32% <0.00%> (-0.58%)	⬇️
...dev/v1alpha2/application/application_controller.go	84.41% <0.00%> (ø)
... and 10 more

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[Somefive]

Do we need backport this PR to previous version? /cc @wangyikewxgm

[wangyikewxgm]

Yes, this is a general problem.

[github-actions]

Successfully created backport PR #5289 for release-1.6.

[github-actions]

Successfully created backport PR #5290 for release-1.7.