Fix test flakiness due to using inconsistent time. (backport #2485)

[mergify]

Changelist

Use start of hour in tests to fix flaky test issues.

Test Plan

[Describe how this PR was tested (if applicable)]

Author/Reviewer Checklist

• If this PR has changes that result in a different app state given the same prior state and transaction list, manually add the state-breaking label.
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Summary by CodeRabbit

• Bug Fixes
  • Adjusted the calculation of blockTime for specific PnlTicks, ensuring accurate aggregation based on hourly timestamps.

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This is an automatic backport of pull request #2485 done by [Mergify](https://mergify.com).