input: Write a python function to repeat a given string n times.
  assert repeat_string("Hello", 4) == "HelloHelloHelloHello"
  assert repeat_string("abc", 3) == "abcabcabc"
  assert repeat_string("xyz", 1) == "xyz"
  assert repeat_string("test", 0) == ""
output: def repeat_string(s, n):
      return s * n