tests: row_cache: Improve test_mvcc()
tests: row_cache: Improve test_mvcc()

assert_that().is_equal_to() gives better error message.

Also, there is code which can be replaces with
assert_that_stream().has_monotonic_positions()

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