Add testing documentation (#1631)

docs/input/cli/unit-testing.md

@@ -0,0 +1,115 @@
+Title: Unit Testing
+Order: 14
+Description: Instructions for unit testing a Spectre.Console application.
+Reference: 
+    - T:Spectre.Console.Testing.CommandAppTester
+    - T:Spectre.Console.Testing.TestConsole
+    - T:Spectre.Console.Testing.TestConsoleInput
+---
+
+`Spectre.Console` has a separate project that contains test harnesses for unit testing your own console applications. 
+
+The fastest way of getting started is to install the `Spectre.Console.Testing` NuGet package.
+
+```text
+> dotnet add package Spectre.Console.Testing
+```
+
+`Spectre.Console.Testing` is also the namespace containing the test classes.
+
+## Testing a CommandApp
+
+The `CommandAppTester` is a test implementation of `CommandApp` that's configured in a similar manner but designed for unit testing.
+
+The following example validates the exit code and terminal output of a `Spectre.Console` command:
+
+```csharp
+    /// <summary>
+    /// A Spectre.Console Command
+    /// </summary>
+    public class HelloWorldCommand : Command
+    {
+        private readonly IAnsiConsole _console;
+
+        public HelloWorldCommand(IAnsiConsole console)
+        {
+            // nb. AnsiConsole should not be called directly by the command
+            // since this doesn't play well with testing. Instead,
+            // the command should inject a IAnsiConsole and use that.
+
+            _console = console;
+        }
+
+        public override int Execute(CommandContext context)
+        {
+            _console.WriteLine("Hello world.");
+            return 0;
+        }
+    }
+
+    [TestMethod]
+    public void Should_Output_Hello_World()
+    {
+        // Given
+        var app = new CommandAppTester();
+        app.SetDefaultCommand<HelloWorldCommand>();
+
+        // When
+        var result = app.Run();
+
+        // Then
+        Assert.AreEqual(result.ExitCode, 0);
+        Assert.AreEqual(result.Output, "Hello world.");
+    }
+```
+
+## Testing console behaviour
+
+ `TestConsole` and `TestConsoleInput` are testable implementations of `IAnsiConsole` and `IAnsiConsoleInput`, allowing you fine-grain control over testing console output and interactivity.
+
+The following example renders some widgets before then validating the console output:
+
+```csharp
+    [TestMethod]
+    public void Should_Render_Panel()
+    {
+        // Given
+        var console = new TestConsole();
+
+        // When
+        console.Write(new Panel(new Text("Hello World")));
+
+        // Then
+        Assert.AreEqual(console.Output, """"
+┌─────────────┐
+│ Hello World │
+└─────────────┘
+
+"""");
+    }
+```
+
+While `Assert` is fine for validating simple output, more complex output may benefit from a tool like [Verify](https://github.com/VerifyTests/Verify).
+
+The following example prompts the user for input before then validating the expected choice was made:
+
+```csharp
+    [TestMethod]
+    public void Should_Select_Orange()
+    {
+        // Given
+        var console = new TestConsole();
+        console.Input.PushTextWithEnter("Orange");
+
+        // When
+        console.Prompt(
+            new TextPrompt<string>("Favorite fruit?")
+                .AddChoice("Banana")
+                .AddChoice("Orange"));
+
+        // Then
+        Assert.AreEqual(console.Output, "Favorite fruit? [Banana/Orange]: Orange\n");
+    }
+```
+
+`CommandAppTester` uses `TestConsole` internally, which in turn uses `TestConsoleInput`, offering a fully testable harness for `Spectre.Console` widgets, prompts and commands.