Add performance test coverage for AJAX requests

Problem/Motivation

From @catch in 🐛 Aggregate JavaScript files is getting optimized only in maintenance mode Postponed: needs info

Given this was a typo and it's a regression in released versions, I personally think tests should be a follow-up here. Would be good to add to performance test coverage but we'll need an AJAX request that adds at least two JavaScript files at once, and I'm not sure where to find one of those yet.

Steps to reproduce

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Proposed resolution

Add test coverage around the AJAX fix from 🐛 Aggregate JavaScript files is getting optimized only in maintenance mode Postponed: needs info

Remaining tasks

Do it
Review

User interface changes

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Introduced terminology

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API changes

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Data model changes

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Release notes snippet

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