Rollup merge of #145359 - GuillaumeGomez:correctly-pick-search.js, r=lolbinarycat

Fix bug where `rustdoc-js` tester would not pick the right `search.js` file if there is more than one

It happened to me quite a few times recently when I worked on the search index:
1. I make a change in search.js
2. I run `rustdoc-js` tests
3. nothing changes

So my solution was to simply remove the folder, but it's really suboptimal. With this PR, it now picks the most recently modified file.

cc ```@lolbinarycat```

Author: jhpratt