TLS test: Pick available port

[gpotter2]

• Github actions seems to take 59000 on its own. Here's an attempt to stabilize this test.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.15%. Comparing base (65843d0) to head (8044e1f).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4752      +/-   ##
==========================================
- Coverage   82.15%   82.15%   -0.01%     
==========================================
  Files         361      361              
  Lines       87284    87284              
==========================================
- Hits        71711    71708       -3     
- Misses      15573    15576       +3     

see 7 files with indirect coverage changes

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