mutation_test: adjust for column_family_test_config accepting an sstables_manager
mutation_test: adjust for column_family_test_config accepting an sstables_manager

Acquire a test_env and extract an sstables_manager from that, passing it
to column_familty_test_config, in preparation for losing the default
constructor of column_familty_test_config.

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