XCTest dismiss got called tapping UINavigationBar button

ViewControllerTests.swift

class ViewControllerTests: XCTestCase {

  func testDismissGetsCalled() {
    let sut = MockViewController()
    sut.loadViewIfNeeded()

    guard let dismissButton = sut.navigationItem.leftBarButtonItem else { XCTFail("should have a dismiss button"); return }
    guard let action = dismissButton.action else { XCTFail("dismiss button should have an action"); return }
    sut.performSelector(onMainThread: action, with: dismissButton, waitUntilDone: true)
    
    XCTAssertTrue(sut.dismissedGotCalled)
  }

}

extension ViewControllerTests {

    class MockViewController: ViewController {
        var dismissedGotCalled = false
        var completionHandler: (() -> Void)?

        override func dismiss(animated flag: Bool, completion: (() -> Void)? = nil) {
            dismissedGotCalled = true
            completionHandler?()
        }
    }

}