Terminal: measure characters with Unicode 11 widths, so an emoji takes the two columns the remote counts

tests/DodoSSH.Client.Terminal.Tests/RendererUnicodeTests.cs

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+namespace DodoSSH.Client.Terminal.Tests;
+
+/// <summary>
+/// The renderer page measures characters with Unicode 11's widths, so an emoji takes the two columns the
+/// remote counts for it.
+/// </summary>
+/// <remarks>
+/// What is asserted is that the page asks for the table, and asks before the first frame. Whether xterm then
+/// really draws 😀 across two cells is xterm's business, and the fake terminal in the harness is a stub with
+/// a memory, not an emulator; <c>docs/manual-checks.md</c> 1.13 is where a person looks at the real one.
+/// </remarks>
+public sealed class RendererUnicodeTests
+{
+    [Fact]
+    public void APane_LoadsTheUnicode11Addon()
+    {
+        var page = Opened();
+
+        page.HasUnicode11(7).ShouldBeTrue();
+    }
+
+    /// <remarks>
+    /// Before open() and not after it: a table chosen once the pane is open leaves whatever it drew first
+    /// measured with Unicode 6, which is the drift this exists to stop.
+    /// </remarks>
+    [Fact]
+    public void APane_IsOpenedWithUnicode11WidthsAlreadyActive()
+    {
+        var page = Opened();
+
+        page.UnicodeAtOpen(7).ShouldBe("11");
+    }
+
+    private static RendererPage Opened()
+    {
+        var page = RendererPage.Load();
+
+        page.Do("accept(0)");
+        page.Do("openSession(7)");
+
+        return page;
+    }
+}