Add unit test for sequential /set_data requests with random data in src/tests/mod.rs

[Sophie-Ag00001]

# Add Unit Test for Sequential `/set_data` Requests

## Summary
This PR adds a unit test to `src/tests/mod.rs` to verify that sequential requests to the `/set_data` API route with new, random data each time return cumulative results. The test ensures that the response contains all data entries sent in previous requests.

## Motivation
The test was added to validate the behavior of the `/set_data` endpoint when handling multiple sequential requests. This ensures that the API correctly accumulates and returns all data entries, which is critical for maintaining data integrity and consistency.

## Context
The `/set_data` route is expected to store and return all data entries sent in previous requests. This test verifies that the endpoint adheres to this requirement by sending three requests with random data and checking the response.

## Testing Instructions
1. Run the test suite using `cargo test`.
2. Verify that the new test `test_sequential_set_data_requests` passes.
3. Ensure no other tests are affected by this change.

## Known Issues/Limitations
- The test assumes that the `/set_data` endpoint is already implemented and functional.
- Random data generation is limited to the scope of the test and does not cover all edge cases.