Add testing documentation

docs/input/testing/cli.md

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+Title: Testing Spectre CLI Apps
+Description: "Test Spectre CLI features,functionality, output, and performance."
+---
+
+The `CommandAppTester` class is used to test a Spectre CLI application.
+It should be used to test a `CommandApp` `Command`s and `CommandSetting`s.
+
+## CommandAppTester
+
+The `CommandAppTester` class is a wrapper around the `CommandApp` object configured for testing.
+
+### Example
+
+```csharp
+public class Program
+{
+    public static int Main(string[] args)
+    {
+        var app = new CommandApp();
+        app.Configure(appConfig);
+        var result = app.Run(args);
+        return result;
+    }
+
+    public static void appConfig(IConfigurator config)
+    {
+        config.AddCommand<HelloCommand>("hello");
+    }
+}
+
+public class HelloCommand : Command
+{
+    public override int Execute(CommandContext context)
+    {
+        AnsiConsole.WriteLine("Hello world");
+        return 0;
+    }
+}
+
+public class ProgramTests
+{
+    [Fact]
+    public void Should_Return_Hello_World()
+    {
+        // Given
+        var app = new CommandAppTester();
+        app.Configure(Program.appConfig);
+
+        // When
+        var result = app.Run("hello");
+
+        // Then
+        Assert.Multiple(() =>
+        {
+            Assert.Equal(0, result.ExitCode);
+            Assert.Equal("Hello world", result.Output);
+        });
+    }
+}
+```

docs/input/testing/console.md

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+Title: Testing Spectre Console
+Description: "Test Spectre Console features,functionality, output, and performance."
+---
+
+The `TestConsole` class is used to test the output of `AnsiConsole`.
+
+## TestConsole
+
+The `TestConsole` class is a wrapper around the `AnsiConsole` object configured for testing, with no (current) support for input.
+
+## Example
+
+```csharp
+public class ProgramTests
+{   
+    [Fact]
+    public void Should_Return_Hello_World()
+    {
+        // Given
+        var console = new TestConsole();
+        // When
+        console.WriteLine("Hello world");
+        // Then
+        Assert.Equal("Hello world" + Environment.NewLine, console.Output);
+    }
+}
+```

docs/input/testing/index.cshtml

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+Title: Spectre.Console.Testing
+Order: 90
+Description: "The testing suite for the command line interface and spectre console...."
+---
+
+<p>Spectre.Console.Testing is the namespace for testing suite of Spectre Console and CLI.
+    The <a href="https://github.com/VerifyTests/Verify">Verify</a> package can also be used to verify console output.
+</p>
+
+<ul>
+    @foreach(IDocument child in OutputPages.GetChildrenOf(Document))
+    {
+        <li>@Html.DocumentLink(child)</li>
+    }
+</ul>