Add custom dashboard namespace test

[p37ruh4]

Related to #6098 and requested by @floryut here: #6107 (comment)

What type of PR is this?

Uncomment only one /kind <> line, hit enter to put that in a new line, and remove leading whitespaces from that line:

/kind api-change
/kind bug
/kind cleanup
/kind design
/kind documentation
/kind failing-test
/kind feature

/kind flake

What this PR does / why we need it:
Adds custom dashboard naming test

Which issue(s) this PR fixes:

Does not fix the issue but is related to #6098

Special notes for your reviewer:
@floryut wanted to add this test #6107 (comment)

Does this PR introduce a user-facing change?:

NONE

[k8s-ci-robot]

Hi @p37ruh4. Thanks for your PR.

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[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: p37ruh4
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You can assign the PR to them by writing /assign @ant31 in a comment when ready.

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[p37ruh4]

Replaces #6133 as I had issues ammending previous commit message.

[LuckySB]

/ok-to-test
/need-rebase

[floryut]

@p37ruh4 Your git history seems weird, please rebase master and only put your commit (no merge commit, rebase only)

[floryut]

@p37ruh4 Are you still around ?

[p37ruh4]

@floryut, I will create new pull request as this one has both merge conflicts and is outdated.