The workload generator in test/model currently ensures that there are no write conflicts between operations in the workload. We should relax this requirement, so that it would occasionally include operations that conflict with each other.

This would, for example, reproduces issues such as WT-13694 faster. Even though WT-13694 was caught by test/model, it was merely a lucky coincidence that it generated that test case while performing counterexample reduction for a different issue.