In-Memory Cache with Expiration

Design and implement a simple in-memory key-value store that supports automatic expiration of entries. The key-value store should provide at least the following functionalities:

1. Set(key, value, ttl): Insert a key with an associated value and a time-to-live (TTL) in seconds. Once the TTL has expired, the key should no longer be accessible.
2. Get(key): Retrieve the value associated with the key if it exists and has not expired; otherwise, return an appropriate null or error response.
3. Pruning Mechanism: The system should automatically clean up expired keys so that they do not consume memory unnecessarily. You may choose whether this cleanup is done lazily (upon access) or with a background process.

Consider the following requirements:

• The store should be efficient in both time and space.
• Multiple operations (set and get) may occur concurrently. Think about thread safety if you are using a language where it matters.
• It should be easy to test your implementation.

Implement your solution in a programming language of your choice. Include unit tests to demonstrate the functionality of your implementation.