Keep timestamps current for URL Metrics in fixture

[westonruter]

This addresses an oversight from #1903. When I captured the URL Metrics in the fixture, the timestamp values were all newer than 1 week, so they were all considered fresh. However, after a week test_get_lcp_element_when_group_half_stale started to fail because the URL Metrics would never be half stale anymore due to a different ETag: They would be completely stale due to having timestmaps older than the freshness TTL of one week.

[github-actions]

The following accounts have interacted with this PR and/or linked issues. I will continue to update these lists as activity occurs. You can also manually ask me to refresh this list by adding the props-bot label.

If you're merging code through a pull request on GitHub, copy and paste the following into the bottom of the merge commit message.

Co-authored-by: westonruter <westonruter@git.wordpress.org>
Co-authored-by: swissspidy <swissspidy@git.wordpress.org>
Co-authored-by: mukeshpanchal27 <mukesh27@git.wordpress.org>

To understand the WordPress project's expectations around crediting contributors, please review the Contributor Attribution page in the Core Handbook.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 69.62%. Comparing base (1a7f906) to head (7737662).
Report is 2 commits behind head on trunk.

Additional details and impacted files

@@           Coverage Diff           @@
##            trunk    #1913   +/-   ##
=======================================
  Coverage   69.62%   69.62%           
=======================================
  Files          86       86           
  Lines        6990     6990           
=======================================
  Hits         4867     4867           
  Misses       2123     2123           

Flag	Coverage Δ
multisite	69.62% <ø> (ø)
single	39.58% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.