How does the `any Publisher` work in Swift?

Question

I try to create a class implementation skeleton and a test for it. I have already had the function setResult and test testSetResult. Could I have something similar for the function setPublisher and testSetPublisher?

class FakeClass {
  // ...

  public func setResult() -> Result<[String: Any], Error> {
    return .success([:])
  }

  public func setPublisher() -> any Publisher<[String, Any], Error> {
    // what should I return? return PassthroughSubject()
  }
}

class FakeClassTest: XCTestCase {
  // ...
  var fakeClass = DependencyInjectionFakeClass() // some dependency injection
  
  func testSetResult() throws {
    let result = fakeClass.setResult()
    switch result {
    case .success(let response):
      XCTAsserture(response.isEmpty)
    case .failure:
      XCTFail()
  }

  // just a simple fake test
  // How could I implement it or do we have sth similar with the function `testSetResult()`
  func testSetPublisher() throws {
    
  }
}