fix: pass expose-gc via NODE_OPTIONS in test scripts

[mcollina]

Summary

• pass --expose-gc to GC-dependent test runs via NODE_OPTIONS
• stop relying on borp --expose-gc forwarding the flag into the executed test process
• restore CI coverage for the test:unit, test:fetch, test:eventsource, and test:tdd scripts

Testing

• npm run test:unit
• npm run test:fetch
• npm run test:eventsource

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.07%. Comparing base (a516f87) to head (0c05e01).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5054      +/-   ##
==========================================
- Coverage   93.08%   93.07%   -0.02%     
==========================================
  Files         110      110              
  Lines       35804    35804              
==========================================
- Hits        33327    33323       -4     
- Misses       2477     2481       +4     

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