<human>:  add suffix to filename.
<bot>: Here is a Python function to add suffix to filename

import os

def add_suffix(filename, suffix):
  """Add suffix to filename."""
  return filename + suffix

# Enter the string
filename = "test.txt"

# Calling the function
filename = add_suffix(filename, ".txt")

# Printing the string
print(filename)

#Example Output
# test.txt.txt

#Remember to inspect code for bugs.