input: Write a python function to generate a matrix of size NxN filled with a specific number.
  assert mat_gen(3, 5) == [[5, 5, 5], [5, 5, 5], [5, 5, 5]]
  assert mat_gen(2, 1) == [[1, 1], [1, 1]]
  assert mat_gen(4, 0) == [[0, 0, 0, 0], [0, 0, 0, 0], [0, 0, 0, 0], [0, 0, 0, 0]]
output: def mat_gen(N, num):
      return [[num for _ in range(N)] for _ in range(N)]