tests: mvcc: Use the standard maybe_merge_versions() to merge snapshots
tests: mvcc: Use the standard maybe_merge_versions() to merge snapshots

Preparation for switching to background merging.

scylladb/scylla,avikivity/scylla,duarten/scylla,scylladb/scylla,duarten/scylla,duarten/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla