docs: Developer guideline to write unit/integration tests in CAPA repo

[Ankitasw]

What type of PR is this?
/kind documentation

What this PR does / why we need it:
As part of #2910, we have added few integration and unit tests in controllers. We have separated two files *_unit_test.go specifically for unit tests and *_test.go for integration tests.
This PR makes clear distinction between the two files and convention of how to differentiate integration vs unit tests.

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 #3288

Checklist:

• squashed commits
• includes documentation
• adds unit tests
• adds or updates e2e tests

Release note:

Updated the test conventions used for CAPA controllers.

[k8s-ci-robot]

@Ankitasw: This issue is currently awaiting triage.

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[sedefsavas]

/approve

cc @pydctw if there is any comment.

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: sedefsavas

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[Ankitasw]

@sedefsavas @pydctw please let me know if you have any comments here.

[pydctw]

+1 discussing naming conventions during office hours.

[pydctw]

/lgtm