tests: mutation_source_tests: Fix use-after-scope on partition range
tests: mutation_source_tests: Fix use-after-scope on partition range

Message-Id: <f79508ceca2c06bf0040afaa726378eb7672cb05@scylladb.com>

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