Sure! Here's a test case generator for the problem statement you provided. This generator will create random valid inputs for the problem, ensuring that the constraints are respected.

```python
import random
import string

def tcgen():
    # Generate random lengths for S' and T
    len_S = random.randint(1, 50)
    len_T = random.randint(1, min(50, len_S))
    
    # Generate a random string T consisting of lowercase English letters
    T = ''.join(random.choice(string.ascii_lowercase) for _ in range(len_T))
    
    # Generate a random string S' with lowercase letters and '?'
    S_prime = []
    for _ in range(len_S):
        if random.random() < 0.3:  # 30% chance of being a '?'
            S_prime.append('?')
        else:
            S_prime.append(random.choice(string.ascii_lowercase))
    S_prime = ''.join(S_prime)
    
    return S_prime, T

# Example of generating a test case
S_prime, T = tcgen()
print("S' =", S_prime)
print("T =", T)
```

This function generates:
- A random length `len_S` for the string `S'`.
- A random length `len_T` for the string `T` such that `len_T <= len_S`.
- A random string `T` of length `len_T` consisting of lowercase English letters.
- A random string `S'` of length `len_S` consisting of lowercase English letters and the character '?'.

The function prints an example generated test case. You can call this function multiple times to generate different test cases for your problem.