[php8-compact][phpunit8][NFC] Remove timecop function references from Cache Tiered Test

[seamuslee001]

Overview

This removes the last usage of timecop from CiviCRM unit testing

Before

Timecop used if it is available

After

Timecop not used

ping @totten

[civibot]

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[totten]

Thanks @seamuslee001.

So I think the only impact will be that testTieredTimeout_explicitHigh_lateReoad will run 5 seconds slower - which, for this one E2E test, seems worth it (if it means that we can remove this question from PHP8 support-path).

r-test passes, so let's do it...