prevent overflow in port allocation logic for tests

[alexpyattaev]

Problem

#5865 did not allow for sufficiently high number of nextest slots. CI machines have 64 cores => 64 slots and logic only worked correctly for ~59 slots causing sporadic test failures.

Summary of Changes

• Constraint the number of concurrent tests < 64 (in case we get threadripper CI runners)
• Fix allocation logic to not overflow with 64 slots
• Reduce number of nodes in one gossip test from 50 to 40 (so we can fit in 47 20-port slices available per test process).

Fixes

Flakes like this one https://buildkite.com/anza/agave/builds/22593#01964750-dee6-49c7-8225-5a744952947f

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 25.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 82.9%. Comparing base (86b229b) to head (a8de374).
Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##           master    #5889     +/-   ##
=========================================
- Coverage    82.9%    82.9%   -0.1%     
=========================================
  Files         830      830             
  Lines      376347   376347             
=========================================
- Hits       312282   312281      -1     
- Misses      64065    64066      +1     

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