cql3: expr: Handle subscript in test_assignment
cql3: expr: Handle subscript in test_assignment

test_assignment can't be passed a column_value,
so a subscript won't work as well.

Signed-off-by: Jan Ciolek <b66a6f3989e26cb552db14b7ab163c3a50292e24@scylladb.com>

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