Write a function toggle_bits(s) that takes in a string s of '0's and '1's and returns a string in which each bit in s has been toggled.
A. Develop a function to toggle bits in a binary string.
B. Implement a GUI interface for users to input binary strings and toggle bits.
C. Integrate the toggle_bits function into an existing codebase.
D. Create unit tests to ensure the toggle_bits function behaves as expected.