https://stackoverflow.com/a/66053463/1271826

MyAppTests.swift

final class MyAppTests: XCTestCase {
    var cancellables: Set<AnyCancellable> = []

    func testDemoDataIncorrect() {
        let start = Date.now

        // fire an event in 1 second, but only get the first value

        let publisher = Timer.publish(every: 1, on: .main, in: .default)
            .autoconnect()
            .first()

        publisher.sink { result in
            if case .failure(let error) = result {
                XCTAssert(true, "\(error)")
            }
        } receiveValue: { date in
            XCTAssertLessThan(date, start) // should be XCTAssertGreaterThan(response, start)
        }
        .store(in: &cancellables)
    }

    func testDemoData() {
        let start = Date.now

        let expectation = expectation(description: "timer should fire")

        let publisher = Timer.publish(every: 1, on: .main, in: .default)
            .autoconnect()
            .first()
            .eraseToAnyPublisher()

        publisher.sink { result in
            if case .failure(let error) = result {
                XCTAssert(true, "\(error)")
            }
            expectation.fulfill()
        } receiveValue: { date in
            XCTAssertGreaterThan(date, start)
        }
        .store(in: &cancellables)

        waitForExpectations(timeout: 4)
    }
}

A few changes from the original example:

1. Rather than Just, I use a Timer.TimerPublisher to consistently manifest the scenario where a publisher publishes a value after you have already returned from testDemoDataIncorrect.

2. An expectation can be used to make sure you don't return from testDemoData before the value is published.