tests: Fix time comparison by using UTC timezone

[gogolok]

Is there a related GitHub Issue?

No.

https://cloudfoundry.slack.com/archives/C0297673ASK/p1729171006582719

What is this change about?

When running unit tests locally, some unit tests were failing on machines with local time != UTC.

Here's an extract (some lines left out):

Summarizing 3 Failures:
  [FAIL] Service Offering [It] returns the expected JSON
  .../api/presenter/service_offering_test.go:75
  [FAIL] Service Broker [It] returns the expected JSON
  .../api/presenter/service_broker_test.go:57
  [FAIL] Service Plan ForServicePlan [It] returns the expected JSON
  .../api/presenter/service_plan_test.go:100

expected
  "created_at": "1970-01-01T01:00:01+01:00",
to match JSON of
  "created_at": "1970-01-01T00:00:01Z",
  
  first mismatched key: "created_at"

Does this PR introduce a breaking change?

No.

[linux-foundation-easycla]

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• ✅ login: gogolok / name: Robert Gogolok (9327fc1)