tests: streamed_mutation_assertions: Merge range_tombstones with the same position in produces_range_tombstone()
tests: streamed_mutation_assertions: Merge range_tombstones with the same position in produces_range_tombstone()

In preparation for allowing fragment stream to produce consecutive
range tombstones with the same position.

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