Fix failing tests with WordPress `trunk`

[felixarntz]

Bug Description

See e.g. https://github.com/WordPress/performance/actions/runs/10820446352/job/30020524054?pr=1492: The PHPUnit tests fail, but that's entirely independent of that PR.

It looks like these failures are because we're looking at the exact img tag markup for the "Modern Image Formats" plugin, which of course was changed as of committing sizes="auto" to Core.

Steps to reproduce

You should see those failures as soon as you open any PR, or run unit tests locally against WordPress trunk.

[felixarntz]

@adamsilverstein @mukeshpanchal27 @joemcgill Could one of you please take a look at this, so we can unblock other PRs with these failing tests?

[westonruter]

PR with fix: #1533