test: dispose of filehandles in filehandle.read tests

[LiviaMedeiros]

test-fs-read-empty-buffer.js did not close used filehandle.
test-fs-read-offset-null reused same variable in async tests, creating potential race condition.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.22%. Comparing base (f497881) to head (9499153).
Report is 28 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #58543      +/-   ##
==========================================
- Coverage   90.23%   90.22%   -0.01%     
==========================================
  Files         635      635              
  Lines      187580   187580              
  Branches    36860    36854       -6     
==========================================
- Hits       169265   169253      -12     
- Misses      11101    11115      +14     
+ Partials     7214     7212       -2     

see 28 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/67259/

[nodejs-github-bot]

Landed in 9e35ddc