Fixes #4076 cursor text field (#4077)

* Add test for TextField cursor position

* Add comment and one more assert

* Fix cursor position at the end

* Remove unused local field

---------

Co-authored-by: BDisp <bd.bdisp@gmail.com>

Tests/TerminalGuiFluentTestingXunit/XunitContextExtensions.cs

@@ -1,4 +1,5 @@
-using TerminalGuiFluentTesting;
+using System.Drawing;
+using TerminalGuiFluentTesting;
 using Xunit;
 
 namespace TerminalGuiFluentTestingXunit;
@@ -6,4 +7,27 @@ namespace TerminalGuiFluentTestingXunit;
 public static partial class XunitContextExtensions
 {
     // Placeholder
+
+
+    /// <summary>
+    /// Asserts that the last set cursor position matches <paramref name="expected"/>
+    /// </summary>
+    /// <param name="context"></param>
+    /// <param name="expected"></param>
+    /// <returns></returns>
+    public static GuiTestContext AssertCursorPosition (this GuiTestContext context, Point expected)
+    {
+        try
+        {
+            Assert.Equal (expected, context.GetCursorPosition ());
+        }
+        catch (Exception)
+        {
+            context.HardStop ();
+
+            throw;
+        }
+
+        return context;
+    }
 }