test: Fix flaky subscribe tests (#6510)

Subscribe tests have a problem that there is no way to synchronize application running in background threads and test threads. Threads are communicating via websocket messages. When the code is compiled in debug mode with code coverage enabled it executes quite slow, so receiving websocket messages by the client in subscribe tests may time out.

This change 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, which has also been addressed.

.github/workflows/reusable-build-test-config.yml

@@ -230,6 +230,8 @@ jobs:
           BUILD_NPROC: ${{ steps.nproc.outputs.nproc }}
         run: |
           set -o pipefail
+          # Coverage builds are slower due to instrumentation; use fewer parallel jobs to avoid flakiness
+          [ "$COVERAGE_ENABLED" = "true" ] && BUILD_NPROC=$(( BUILD_NPROC - 2 ))
           ./xrpld --unittest --unittest-jobs "${BUILD_NPROC}" 2>&1 | tee unittest.log
 
       - name: Show test failure summary