fix: Add test cases for validator and manifest generator

[gaocegege]

Signed-off-by: Ce Gao gaoce@caicloud.io

What this PR does / why we need it:

We do not have test for validator and metricscollector template/trial template. This PR is to add the test code about these three parts.

pkg/controller/v1alpha2/experiment/manifest coverage: 71.9% of statements
pkg/controller/v1alpha2/experiment/validator coverage: 73.9% of statements

What I do in this PR:

• Make kubeclient an interface instead of a struct, and generate a mock implementation for it.
• Make metricscollector template (manifest-util.go) and trial template (template-util.go) related logic an interface (producer interface), instead of several functions in util package, generate mock for it, write the unit test for it.
• Make validator logic (webhook-util.go) in an interface, and write the unit test for it. Since it is only used in validation webhook, we do not mock it.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #

Special notes for your reviewer:

1. Please confirm that if this PR changes any image versions, then that's the sole change this PR makes.

Release note:

---

This change is 

[gaocegege]

Blocked by #512

[gaocegege]

/hold

[gaocegege]

/hold cancel
/cc @johnugeorge @hougangliu

[gaocegege]

/retest

[gaocegege]

/cc @johnugeorge

[gaocegege]

/retest

[gaocegege]

PTAL @hougangliu @johnugeorge

[hougangliu]

I think namespace and experiment param can be removed since we can get them by e *experimentsv1alpha2.Experiment

[hougangliu]

@gaocegege I like this PR. LGTM, and only one mini comment

[gaocegege]

/approve

@hougangliu I need your approval to merge the PR. I think your suggestion is reasonable. I will fix it in another PR. Thanks.

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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[hougangliu]

/lgtm