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

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.75%. Comparing base (e15bbda) to head (6896452).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5878      +/-   ##
==========================================
+ Coverage   89.71%   89.75%   +0.04%     
==========================================
  Files          64       64              
  Lines        4695     4694       -1     
  Branches      978      978              
==========================================
+ Hits         4212     4213       +1     
+ Misses        483      481       -2     

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