A reviewer noted that test_update_expression_list_append_non_list_arguments
has too much code duplication - the same long API call to run
"SET a = list_append(...)" was repeated many times.
So in this patch we add a short inner function "try_list_append" to
avoid this duplication.
Signed-off-by: Nadav Har'El <nyh@scylladb.com>
Closes: #15298