Make the following program with TDD and Golang
Write a function combined_number accepting a list of non negative integers, and returning their largest possible combined number as a string. For example
given [50, 2, 1, 9] it returns "95021" (9 + 50 + 2 + 1)
given [5, 50, 56] it returns "56550" (56 + 5 + 50)
given [420, 42, 423] it returns "42423420" (42 + 423 + 420)