// ovo ne radi
private func insert(_ data: Data, to sut: SUT) -> Error? {
    let exp = expectation(description: "Wait for cache insertion")
    var insertionError: Error?
    sut.insert(data) { receivedInsertionError in
        insertionError = receivedInsertionError
        exp.fulfill()
    }
    wait(for: [exp], timeout: 3)
    return insertionError
}

// ovo radi
private func insert(_ data: Data, to sut: SUT, completion: ((_ error: Error?) -> Void)? = nil){
    let exp = expectation(description: "Wait for cache insertion")
    var insertionError: Error?
    sut.insert(data) { receivedInsertionError in
        insertionError = receivedInsertionError
        exp.fulfill()
    }
    wait(for: [exp], timeout: 3)
    completion?(insertionError)
}