test: reduce retry-after invalid date flake

[mcollina]

Summary

• use monotonic timing and tolerance for the invalid Retry-After date test
• avoids wall-clock jitter causing false failures

Testing

• npx borp -p "test/retry-handler.js"

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.25%. Comparing base (352c691) to head (4a05f28).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4788   +/-   ##
=======================================
  Coverage   93.25%   93.25%           
=======================================
  Files         109      109           
  Lines       34024    34024           
=======================================
  Hits        31729    31729           
  Misses       2295     2295           

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[mertcanaltin]

Lgtm