input: Write a python function to combine two arrays into one.
  assert combine_arrays([1, 2, 3], [4, 5, 6]) == [1, 2, 3, 4, 5, 6]
  assert combine_arrays([7, 8], [9, 10, 11]) == [7, 8, 9, 10, 11]
  assert combine_arrays([], [1, 2, 3]) == [1, 2, 3]
output: def combine_arrays(arr1, arr2):
      return arr1 + arr2