[WP6.8] Fix 'wp-polyfill' script dependents unit test

[t-hamano]

What?

Similar to #69912

Fix 'wp-polyfill' script dependents unit test failure occurring on wp/6.8 branch.

Why?

This unit test detects which other scripts depend on the wp-polyfill script and verifies that the list matches our expectations.

The wp-fields script was unintentionally bundled with WP 6.7, even though it should never have been included in core.

In addition, the unit test is executed on previous WordPress versions too. This means that in the trunk and wp/6.8 branches, unit tests will be executed on WP 6.7 too.

Because WP 6.7 unintentionally includes the wp-fields script, we should ignore wp-fields in the wp/6.8 branch, just like we would in the trunk branch.

Testing Instructions

Let's see if CI checks are green.

[t-hamano]

@Mamaduka I would like to merge this PR to unblock other PRs that need to be backported to the wp/6.8 branch.

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