Add human-readable eval_time to rules unit tests

[kustodian]

Proposal

Rules unit tests would be much easier to read if the eval_time supported human-readable times like this:

    alert_rule_test:
        # Alert should not be triggered after 3h and 59m
      - eval_time: 3h59m  # instead of 239m
        alertname: postgres_invalid_indices
        exp_alerts: []

        # Alert should not be triggered after 4h and 6m (because of staleness)
      - eval_time: 4h6m  # instead of 246m
        alertname: postgres_invalid_indices
        exp_alerts: []

If something needs to be triggered after 4 hours and if we want to test will it be triggered at 3 hours and 59 minutes, it's easier to read 3h59m, then 239m. Samo goes for 4h6m instead of 246m.

[rudo-thomas]

Durations like 3h59m used to be supported before 2.20.0, see this PR (found through the 2.20.0 release notes).

[roidelapluie]

We now support them again: #7713 . Note that we do not support non-integers (1.5h).