Fix intermittent failing test helper

[jjngx]

Proposed changes

Remove parallelisation from the healthcheck package tests.

The parallelisation is removed to avoid the intermittent port binding issues in the test helper responsible for staring a new test server.

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works
• I have checked that all unit tests pass after adding my changes
• I have updated necessary documentation
• I have rebased my branch onto main
• I will ensure my PR is targeting the main branch and pulling from my branch from my own fork

[codecov]

Codecov Report

Merging #4432 (0183040) into main (9dcd4cc) will decrease coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #4432      +/-   ##
==========================================
- Coverage   52.12%   52.09%   -0.03%     
==========================================
  Files          59       59              
  Lines       16956    16956              
==========================================
- Hits         8838     8834       -4     
- Misses       7821     7823       +2     
- Partials      297      299       +2     

see 1 file with indirect coverage changes

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