Sure, I can help you write a test case generator for the given problem statement. Here's a Python function to generate random test cases for the problem:

```python
import random

def tcgen():
    # Length of the string |s| between 1 and 200000
    length = random.randint(1, 200000)
    
    # Generate a random string of length `length` consisting of 'A', 'B', 'C'
    s = ''.join(random.choice('ABC') for _ in range(length))
    
    return s

# Example usage
if __name__ == "__main__":
    for _ in range(5):
        print(tcgen())
```

Explanation:

1. We first determine the length of the string `s`, which is a random integer between 1 and 200000.
2. We then generate a random string of the determined length, where each character is randomly chosen from the set {'A', 'B', 'C'}.
3. The generated string `s` is returned as the test case.

This function will help you generate different test cases with varying lengths and compositions of 'A', 'B', and 'C'. You can adjust the number of test cases to be generated by changing the range in the example usage section.