Add integration tests for RunnerType.upgradeStatus

What does this MR do and why?

Describe in detail what your merge request does and why.

This MR adds:

• an integration test between the GraphQL RunnerType.upgradeStatus field and the Gitlab::Ci::RunnerUpgradeCheck class, to uncover situations like #367829 (closed) during development;
• an integration test between Ci::Runners::ReconcileExistingRunnerVersionsService#execute and the Gitlab::Ci::RunnerUpgradeCheck class for good measure.

Part of #367836 (closed)

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How to set up and validate locally

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1. Running the new test without the changes in !92411 (merged):

   

2. Running the new test with the changes results in green tests.

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