tests/mutation_reader_test: refactor combined_mutation_reader_test
tests/mutation_reader_test: refactor combined_mutation_reader_test

Make combined_mutation_reader_test more interesting:
* Set the levels on the sstables
* Arrange the sstables so that they test for the "jump over sstables"
bug.
* Arrange the sstables so that they test for the "gap between sstables".

While at it also make the code more compact.

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