Update trailing code and add unit tests

Author: Ecarrion

WooCommerce/Classes/ViewRelated/Orders/Order Creation/NewOrderViewModel.swift

@@ -549,13 +549,13 @@ private extension NewOrderViewModel {
     /// Calculates what navigation trailing item should be shown depending on our internal state.
     ///
     func configureNavigationTrailingItem() {
-        Publishers.CombineLatest(orderSynchronizer.orderPublisher, $performingNetworkRequest)
-            .map { [weak self] order, performingNetworkRequest -> NavigationItem in
-                guard let self = self, !performingNetworkRequest else {
+        Publishers.CombineLatest3(orderSynchronizer.orderPublisher, $performingNetworkRequest, Just(flow))
+            .map { order, performingNetworkRequest, flow -> NavigationItem in
+                guard !performingNetworkRequest else {
                     return .loading
                 }
 
-                switch self.flow {
+                switch flow {
                 case .creation:
                     return .create
                 case .editing:

WooCommerce/WooCommerceTests/ViewRelated/Orders/Order Creation/NewOrderViewModelTests.swift

@@ -22,6 +22,17 @@ final class NewOrderViewModelTests: XCTestCase {
         XCTAssertEqual(viewModel.productRows.count, 0)
     }
 
+    func test_edition_view_model_has_a_navigation_done_button() {
+        // Given
+        let stores = MockStoresManager(sessionManager: .testingInstance)
+
+        // When
+        let viewModel = NewOrderViewModel(siteID: sampleSiteID, flow: .editing(initialOrder: .fake()), stores: stores)
+
+        // Then
+        XCTAssertEqual(viewModel.navigationTrailingItem, .done)
+    }
+
     func test_loading_indicator_is_enabled_during_network_request() {
         // Given
         let stores = MockStoresManager(sessionManager: .testingInstance)