Testing aync `combine` code

TestSwiftCombineCode.swift

    protocol DollarRateUseCase {
        func fetchDollarRate() -> AnyPublisher<Int, Never>
    }



    struct DollarRateUseCaseTask: DollarRateUseCase {
        func fetchDollarRate() -> AnyPublisher<Int, Never> {
            return Future { promise in
                DispatchQueue.main.asyncAfter(deadline: .now() + 5) {
                    promise(.success(83))
                }
            }.eraseToAnyPublisher()
        }
    }


    struct DollarRateUseCaseMockTask: DollarRateUseCase {
        func fetchDollarRate() -> AnyPublisher<Int, Never> {
            return Just<Int>(80) .eraseToAnyPublisher()
        }
    }



    class ViewModel {
        var useCase: DollarRateUseCase
        init(useCase: DollarRateUseCase = DollarRateUseCaseTask()) {
            self.useCase = useCase
        }
        
        func fetch() -> AnyPublisher<Int, Never> {
            return useCase.fetchDollarRate()
        }
    }
    
    // Testing 
     final class TestDollarRateSpec: XCTestCase {
        let viewModel = ViewModel(useCase: DollarRateUseCaseMockTask())
        var cancellable =  Set<AnyCancellable>()
        func testFetchingDollarRate() {
            let exp = expectation(description: "await completion")
            viewModel.fetch().sink { value in
                XCTAssertTrue(value == 80, "Getting failed to replicate mock dollar rate ")
                exp.fulfill()
            }.store(in: &cancellable)
            wait(for: [exp], timeout: 2)
        }
    }