Added & modified unit tests (#2013)

Author: kubaflo

src/Controls/tests/Core.UnitTests/NavigationProxyTests.cs

@@ -1,3 +1,4 @@
+using System;
 using System.Collections.Generic;
 using System.Linq;
 using System.Threading.Tasks;
@@ -99,17 +100,28 @@ public void Constructor()
 		}
 
 		[Fact]
-		public async Task PushesIntoNextInner()
+		public async Task PushesWithoutInner()
 		{
 			var page = new ContentPage();
 			var navProxy = new NavigationProxy();
 
-			await navProxy.PushAsync(page);
+			Assert.ThrowsAsync<InvalidOperationException>(async () => await navProxy.PushAsync(page));
+		}
 
-			var navTest = new NavigationTest();
-			navProxy.Inner = navTest;
+		[Fact]
+		public async Task PushesIntoNextInner()
+		{
+			var proxy = new NavigationProxy();
+			var inner = new NavigationTest();
+			var page1 = new ContentPage();
+			var page2 = new ContentPage();
+
+			proxy.Inner = inner;
+
+			await proxy.PushAsync(page1);
+			await proxy.PushAsync(page2);
 
-			Assert.Equal(page, navTest.LastPushed);
+			Assert.Equal(page2, inner.LastPushed);
 		}
 
 		[Fact]

src/Controls/tests/Core.UnitTests/NavigationUnitTest.cs

@@ -839,6 +839,15 @@ public async Task PopModalWithEmptyStackThrows()
 			Assert.ThrowsAsync<InvalidOperationException>(() => window.Navigation.PopModalAsync());
 		}
 
+		[Fact]
+		public async Task PushWithoutWrappingIntoNavigationPage()
+		{
+			var window = new TestWindow(new ContentPage());
+			var contentPage1 = new ContentPage();
+			var navigationPage = new TestNavigationPage(true, contentPage1);
+			Assert.ThrowsAsync<InvalidOperationException>(() => window.Navigation.PushAsync(contentPage1));
+		}
+
 		[Fact]
 		public async Task TabBarSetsOnFlyoutPageInsideModalPage()
 		{