Sure, I can help you write a test case generator for the given problem statement "What are the popular stalls?". Here is the Python code for the test case generator:

```python
import random

def generate_test_case():
    test_cases = []
    
    # Number of datasets
    num_datasets = random.randint(1, 100)
    
    for _ in range(num_datasets):
        dataset = []
        total_sales = []
        stores = ['A', 'B', 'C', 'D', 'E']
        
        for store in stores:
            s1 = random.randint(1, 10000)
            s2 = random.randint(1, 10000)
            total = s1 + s2
            dataset.append((s1, s2))
            total_sales.append((total, store))
        
        # Ensure no two stores have the same total sales
        total_sales = list(set(total_sales))
        
        # Sort to find the highest sale
        total_sales.sort(reverse=True)
        
        test_cases.append((dataset, total_sales[0]))
    
    # Convert test cases to input-output format
    input_str = ""
    output_str = ""
    
    for dataset, highest_sale in test_cases:
        for s1, s2 in dataset:
            input_str += f"{s1} {s2}\n"
        output_str += f"{highest_sale[1]} {highest_sale[0]}\n"
    
    input_str += "0 0\n"  # End of input
    
    return input_str.strip(), output_str.strip()

# Example usage
input_str, output_str = generate_test_case()
print("Generated Input:")
print(input_str)
print("\nExpected Output:")
print(output_str)
```

### Explanation:
1. **Number of datasets**: Randomly determine the number of datasets (between 1 and 100).
2. **Dataset Creation**: For each dataset:
   - Create an array for each store ('A', 'B', 'C', 'D', 'E') containing random sales values for the morning (`s1`) and afternoon (`s2`) sessions.
   - Calculate the total sales for each store.
   - Ensure no two stores have the same total sales by using a set.
3. **Sort and Find Highest Sale**: Sort the total sales in descending order to determine the store with the highest sales.
4. **Format for Output**: Convert the datasets into the required input and output format.
5. **End of Input**: Append `0 0` to mark the end of input.

You can run the `generate_test_case()` function to generate a new test case each time.