tests: perf_fast_forward: massage result of sprint() into std::string
tests: perf_fast_forward: massage result of sprint() into std::string

sprint() returns std::string(), but the new format() returns an sstring. Usually
an sstring is wanted but in this case an sstring will fail as it is added to
an std::string.

Fix the failure (after spring->format conversion) by converting to an std::string.

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