Increase number of writes in concurrency test to ensure conflict

[eddyashton]

We've seen a single failure of this test when backporting a recent locking change.

https://dev.azure.com/MSRC-CCF/CCF/_build/results?buildId=87860&view=logs&j=b6a3fffa-f95e-580b-9bcb-4ae7d4681578&t=7439823f-f6a2-5514-9be9-467f4c602fdd&l=145

test 6
      Start  6: kv_test

6: Test command: /CCF/build/kv_test
6: Environment variables: 
6:  TSAN_OPTIONS=suppressions=/CCF/tsan_env_suppressions
6:  ASAN_OPTIONS=alloc_dealloc_mismatch=0
6: Test timeout computed to be: 10000000
6: [doctest] doctest version is "2.4.11"
6: [doctest] run with "--help" for options
6: ===============================================================================
6: ../src/kv/test/kv_contention.cpp:253:
6: TEST SUITE: concurrency
6: TEST CASE:  get_version_of_previous_write ordering
6: 
6: ../src/kv/test/kv_contention.cpp:334: ERROR: CHECK( conflict_count > 0 ) is NOT correct!
6:   values: CHECK( {?} >  0 )
6: 
6: 2024-11-12T17:52:56.497726Z        100 [fail ] ../src/kv/committable_tx.h:253       | Error during serialisation
6: 2024-11-12T17:52:56.585602Z        100 [info ] ../src/kv/test/kv_test.cpp:3013      | Seed: 172319737
6: ===============================================================================
6: [doctest] test cases:   57 |   56 passed | 1 failed | 0 skipped
6: [doctest] assertions: 2901 | 2900 passed | 1 failed |
6: [doctest] Status: FAILURE!
 6/59 Test  #6: kv_test ..........................***Failed    7.50 sec

We expected a conflict, but saw none. Local runs produce hundreds of conflicts. Can repro this by pinning the test to a single core:

$ taskset -c 0 ./kv_test 
[doctest] doctest version is "2.4.11"
[doctest] run with "--help" for options
2024-11-13T10:32:26.372950Z        100 [info ] ../src/kv/test/kv_contention.cpp:336 | Found 0 conflicts
===============================================================================
../src/kv/test/kv_contention.cpp:253:
TEST SUITE: concurrency
TEST CASE:  get_version_of_previous_write ordering

../src/kv/test/kv_contention.cpp:337: ERROR: CHECK( conflict_count > 0 ) is NOT correct!
  values: CHECK( {?} >  0 )

2024-11-13T10:32:26.471539Z        100 [fail ] ../src/kv/committable_tx.h:253       | Error during serialisation
2024-11-13T10:32:26.563254Z        100 [info ] ../src/kv/test/kv_test.cpp:3013      | Seed: 718927790
===============================================================================
[doctest] test cases:    57 |    56 passed | 1 failed | 0 skipped
[doctest] assertions: 14372 | 14371 passed | 1 failed |
[doctest] Status: FAILURE!

So maybe, on the 4-core ACI boxes, we don't run for long enough to see any actual conflicts. This mostly just bumps up the number of writes.

Raised #6640 as a sensible follow-up.

[achamayou]

@eddyashton we should backport this to 5.x to make sure the test is stable there too.