chore: deduplicate subDomainName generation logic in tests

[inteon]

What this PR does / why we need it:
There were a lot of subdomain generation implementations in the tests, now it is centralized. (part of #4128)

Release note:

[jetstack-bot]

Hi @inteon. Thanks for your PR.

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[jetstack-bot]

@inteon: The label(s) kind/chore cannot be applied, because the repository doesn't have them.

In response to this:

/kind chore

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[JoshVanL]

Some comments from me 🙂
/assign @inteon

[JoshVanL]

/approve
/ok-to-test

[jetstack-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: inteon, JoshVanL

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[jakexks]

/lgtm