test: Fix flaky subscribe tests

[kuznetsss]

High Level Overview of Change

This PR hopefully fixes flaky Subscribe tests.
Fixes #6399.

Context of Change

Subscribe tests have a problem that there is no way to synchronise application running in background threads and test's thread. Threads are communicating by websocket messages. When the code is compiled in debug mode with code coverage it executes quite slow, so receiving websocket messages by the client in subscribe tests may time out.
This fix does 2 things to fix the problem:

• increases timeout for receiving a websocket message
• decreases the number of tests running in parallel

While testing the fix for subscribe test another flaky test in ledger replay was found. I left a comment in the test describing what was wrong in the flaky ledger replay test.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Refactor (non-breaking change that only restructures code)
• Performance (increase or change in throughput and/or latency)
• Tests (you added tests for code that already exists, or your new feature included in this PR)
• Documentation update
• Chore (no impact to binary, e.g. .gitignore, formatting, dropping support for older tooling)
• Release

API Impact

• Public API: New feature (new methods and/or new fields)
• Public API: Breaking change (in general, breaking changes should only impact the next api_version)
• libxrpl change (any change that may affect libxrpl or dependents of libxrpl)
• Peer protocol change (must be backward compatible or bump the peer protocol version)

[kuznetsss]

CI failed after running tests 15 times in debug with code coverage. But the other test turned out to be flaky:

19> #212 failed: LedgerReplay_test.cpp(1133)

https://github.com/XRPLF/rippled/actions/runs/22867192993/job/66337167696#step:20:52692

I've added a fix for that. Let's see how it goes now.

[kuznetsss]

With the fix for LedgerReplay test, CI has successfully run tests 7 times and then the runner lost communication.
https://github.com/XRPLF/rippled/actions/runs/22905275224/job/66462220709?pr=6510

Let's try again.

[kuznetsss]

Operation got cancelled in CI. But it've run tests successfully 11 more times. So I think the fix is fine. Overall we have:

• The fix for subscribe test was checked 34 times
• The fix for ledger replay test was checked 18 times

I think this should be enough to say that both fixes are good.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.8%. Comparing base (7b3724b) to head (b6b0dce).
⚠️ Report is 1 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #6510     +/-   ##
=========================================
- Coverage     79.8%   79.8%   -0.0%     
=========================================
  Files          862     862             
  Lines        67883   67883             
  Branches      7551    7549      -2     
=========================================
- Hits         54199   54196      -3     
- Misses       13684   13687      +3     

see 6 files with indirect coverage changes

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[a1q123456]

It's an attempt anyway so it's fine as long as we don't try to cover the issue. i.e. skip the checks if we receive no message. Also I did want to test if NPROC is contributing to the issue. Let's see if things are improved!