tests: perf_fast_forward: Extract json_output_writer::write_common_test_group()
tests: perf_fast_forward: Extract json_output_writer::write_common_test_group()

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