chore: concurrency tests for post with threads

[tanmaykm]

Updated concurrency tests to do post requests too, and use @spawn along with @async.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.71%. Comparing base (7adeb8b) to head (1f1f161).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #262      +/-   ##
==========================================
- Coverage   91.27%   90.71%   -0.57%     
==========================================
  Files           6        6              
  Lines         711      711              
==========================================
- Hits          649      645       -4     
- Misses         62       66       +4     

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