test: assert write return values in buffer-bigint64

[nektro]

https://nodejs.org/api/buffer.html#bufwritebigint64bevalue-offset

Returns: offset plus the number of bytes written.

add more cases that assert this

discovered by oven-sh/bun#17694

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.25%. Comparing base (e03af77) to head (5bc1af9).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #57212      +/-   ##
==========================================
- Coverage   90.27%   90.25%   -0.02%     
==========================================
  Files         630      630              
  Lines      184634   184634              
  Branches    36129    36125       -4     
==========================================
- Hits       166674   166638      -36     
- Misses      11009    11031      +22     
- Partials     6951     6965      +14     

see 36 files with indirect coverage changes

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/65437/