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

```python
import random

def tcgen():
    # Constants for the problem
    MAX_N = 70
    MAX_K = 70
    MOD = 10**9 + 7
    
    # Generate random N and K within the specified range
    N = random.randint(1, MAX_N)
    K = random.randint(1, MAX_K)
    
    # Generate a random string s of length K consisting of 'r' and 'b'
    s = ''.join(random.choice(['r', 'b']) for _ in range(K))
    
    return N, K, s

# Example usage of the generator
N, K, s = tcgen()
print(N, K)
print(s)
```

This function generates random values for `N` and `K` within their respective ranges and constructs a random string `s` of length `K` composed of 'r' and 'b'. Each call to `tcgen()` produces a different test case.

You can call this function and print the test cases to use them in your competitive programming environment. Below is an example of how to use this generator:

```python
if __name__ == "__main__":
    for _ in range(5):  # Generate 5 random test cases
        N, K, s = tcgen()
        print(f"Generated Test Case:")
        print(f"N: {N}, K: {K}")
        print(f"s: {s}")
        print()
```

You can adjust the number of test cases generated by changing the range in the for loop. Each generated test case will be different due to the randomness involved in the generation process.