test: memory_footprint: Avoid invalid identifiers as columnnames
test: memory_footprint: Avoid invalid identifiers as columnnames

Column name should not start with a digit, as can be the case with
random_string().

Message-Id: <f81338307bfb58e51a1363cfd73aef1b8cc21fa7@scylladb.com>

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