The test expects a split to occur but sometimes failed because the 
split check was triggered too early (before the DB had enough keys) 
and found no split key. Once the first split check finds nothing, 
subsequent checks are delayed until `size_diff_hint` (an approximate 
measure of region size change) reaches the `region_split_check_diff` 
threshold. By lowering this threshold, the second split check is 
triggered in time to meet the test's expectation.

In prod, `region_split_check_diff` is typically set to 1/16 of the 
`region_split_size`. For this test, setting it equal to 
`region_split_size` was sufficient to fix the test flakiness.

for i in {1..500}; do                                                
  echo "Run #$i"
  cargo test --package tests --test failpoints --features failpoints --features testexport -- cases::test_split_region::test_split_region_with_no_valid_split_keys --exact --show-output || break
done

None