test/lib/mutation_source_test: test-monotonic-positions: test the reader-under-test
test/lib/mutation_source_test: test-monotonic-positions: test the reader-under-test

Instead of always testing `flat_mutation_reader_from_mutations()`.

Tests: unit(dev, debug)

Signed-off-by: Botond Dénes <a584fa4d75a0a85d721f44fb6e06315a2853b0b6@scylladb.com>
Message-Id: <20200812073406.1681250-1-a584fa4d75a0a85d721f44fb6e06315a2853b0b6@scylladb.com>

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