[tests] require fileutils explicitly

[mtasaka]

ruby 3.4 no longer loads fileutils implicitly.
Fixes #124 .

[apjanke]

Interesting. Is there any documentation on Ruby 3.4 no longer loading fileutils implicitly? I went googling and can't find any. I don't have a local Ruby 3.4 test environment set up yet; hopefully later today.

I'm inclined to just merge this even if that change isn't documented, because I don't see a downside, and the tests are passing. (The one failure is an unrelated rubocop complaint, shown here.)

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[apjanke]

If you rebase on main, I think that'll clear up the "failing checks" issue. I think it's just a rubocop complaint that has been fixed on main.

[mtasaka]

(Just noting that I've rebased the patch)

[apjanke]

All checks pass after the rebase, and this looks good to me. Merged.

[apjanke]

Do you need me to make a new ronn-ng release to make this fix available to you, or is having it on the main branch good enough for now?