The random seed is still not properly tested: I didn't do much about it again, but after a few tries I could still submit my solution.

Needs random tests.

Also note that your test for error handling is not written correctly: The test will pass with at least one call throwing an exception, and not both.

Duplicate of https://www.codewars.com/kata/decimal-to-factorial-and-back

Wouldn't it make more sense if the intoFactorial function took an int and returned a String?
As it stands, it is returning an int whose decimal representation happens to coincide with the factorial base representation of the input, but they are really different numbers.