tests: mutation_source_test: Make blobs more easily distinguishable
tests: mutation_source_test: Make blobs more easily distinguishable

It's easier to compare them if they differ only by a few most
significant bits, than by all bits.

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