fix: run all .only tests across different nesting levels

[mostafaNazari702]

PR Checklist

• Addresses an existing open issue: fixes 🐛 Bug: Multiple .only should run all targeted tests even in a different context #4836
• That issue was marked as status: accepting prs
• Steps in CONTRIBUTING.md were taken

Overview

filterOnly() had a bug where using .only at different nesting levels
would cause nested .only tests to get dropped. For example:

describe('parent', function() {
    it.only('top-level', function() {});
    describe('child', function() {
        it.only('nested', function() {});
    });
});

Only "top-level" woud run -- "nested" was silently discarded because the
_onlyTests branch set this.suites = [], wiping out all child suites
regardless of weither they had .only content.

the fix moves the suite filtering logic out of the else branch so it always
runs. Child suites without any .only content still get pruned correctly.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 63.06%. Comparing base (5b27e3a) to head (a1ba87c).

Files with missing lines	Patch %	Lines
lib/suite.js	0.00%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5878      +/-   ##
==========================================
+ Coverage   63.05%   63.06%   +0.01%     
==========================================
  Files          64       64              
  Lines        4942     4941       -1     
  Branches     1120     1120              
==========================================
  Hits         3116     3116              
+ Misses       1826     1825       -1     

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[mark-wiemer]

No major complaints at a glance, and the test failure is flaky. Thank you :) it will be some time before we're able to review all these PRs, we are focusing on converting from CJS to ESM right now to release Mocha 12 :)

[JoshuaKGoldberg]

Yup, this looks great. Nicely done! 🔥

As Mark said, we're focusing on getting v12 out the door with minimal breaking changes. And this is a semver-major change. Which means it'll have to wait for v13.