test: load the mocha-error esm-utils fixture as .cts

[mostafaNazari702]

PR Checklist

• Addresses an existing open issue: fixes 🛠️ Repo: esm-utils test fails on newer Node 24 because a .ts fixture uses require() #6156
• [] That issue was marked as status: accepting prs
• Steps in CONTRIBUTING.md were taken

Overview

An issue that i filed: #6156, we fix here by renaming the mock-mocha-forbidden-exclusivity-err fixture from .ts to .cts.

fixture uses require(), and after #6078 changed the root package.json to "type": "module", Node started loading the .ts file as ESM, that made "require" unavailable so the test failed with a ReferenceError instead of ERR_MOCHA_FORBIDDEN_EXCLUSIVITY.

Having it as .cts keeps it as CommonJS regardless of the root type setting and we preserve the same require() path the test was added to cover in PR #5647.

Worth noting, found that the Node versions that pinned is not what hides this issue, it fails on newer versions as well, the reason CI passes is nyc, its .ts hook loads the fixture as CommonJS. Running the spec directly skips that hook and exposes the bug.

I tested and now it passes now with+without nyc.

[mostafaNazari702]

[github-actions]

👋 Hi @mostafaNazari702, thanks for the pull request! A scan flagged a concern with it. Could you please take a look?

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.64%. Comparing base (f2243a6) to head (8a10e1e).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6160   +/-   ##
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  Coverage   82.64%   82.64%           
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  Files          62       62           
  Lines        4668     4668           
  Branches     1003     1034   +31     
=======================================
  Hits         3858     3858           
  Misses        810      810           

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