test_runner: fix coverage report when a directory is named file

[heathdutton]

The coverage tree traversal checked tree[key].file to detect file
entries. When a directory named "file" contained a file also named
"file", this check incorrectly matched the child entry instead of
file metadata, causing a TypeError when accessing .path.

Check for .file?.path instead to correctly identify file metadata.

Fixes: #61080

[nodejs-github-bot]

Review requested:

• @nodejs/test_runner

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/70601/

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 88.54%. Comparing base (5e677d6) to head (bc0ee1a).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/internal/test_runner/utils.js	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #61169      +/-   ##
==========================================
+ Coverage   88.52%   88.54%   +0.01%     
==========================================
  Files         703      703              
  Lines      208585   208585              
  Branches    40225    40231       +6     
==========================================
+ Hits       184650   184689      +39     
+ Misses      15953    15909      -44     
- Partials     7982     7987       +5     

Files with missing lines	Coverage Δ
lib/internal/test_runner/utils.js	62.02% <0.00%> (+0.75%)	⬆️

... and 35 files with indirect coverage changes

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