test/boost/mutation_test: add rebuild test for mutation::consume()
test/boost/mutation_test: add rebuild test for mutation::consume()

In the next patches we will refactor mutation::consume(). Before doing
that add another test, which rebuilds the consumed mutation, comparing
it with the original.

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla