Plugin-registerable self tests

The goal of this change is to let plugins register their own self-tests. 

We do this through the `ISelfTestRegistry` interface. For a plugin it
would look like this:

```csharp
[PluginService]
public ISelfTestRegistry SelfTestRegistry

// Somewhere that gets called by your plugin
SelfTestRegistry.RegisterTestSteps([
  new MySelfTestStep(),
  new MyOtherSelfTestStep()
])
```

Where `MySelfTest` and `MyOtherSelfTest` are instances of
the existing `ISelfTestStep` interface.

The biggest changes are to `SelfTestWindow` and the introduction of
`SelfTestWithResults`. I wanted to make sure test state wasn't lost when
changing the dropdown state and I was finding it a bit annoying to work
with the Dictionary now that we can't just rely on the index of the
item.

To fix this I moved all the "test run" state into `SelfTestWithResults`,
most of the changes to `SelfTestWindow` are derived from that, other
then the addition of the combo box.

The documentation for this service is a bit sparse, but I wanted to put
it up for review first before I invest a bunch of time making nice
documentation. 

I'm keen to hear if we think this is useful or if any changes are
needed.

Dalamud/Plugin/SelfTest/ISelfTestRegistry.cs

@@ -0,0 +1,54 @@
+using System.Collections.Generic;
+
+namespace Dalamud.Plugin.SelfTest;
+
+/// <summary>
+/// Interface for registering and unregistering self-test steps from plugins.
+/// </summary>
+/// <example>
+/// Registering custom self-test steps for your plugin:
+/// <code>
+/// [PluginService]
+/// public ISelfTestRegistry SelfTestRegistry { get; init; }
+///
+/// // In your plugin initialization
+/// this.SelfTestRegistry.RegisterTestSteps([
+///     new MyCustomSelfTestStep(),
+///     new AnotherSelfTestStep()
+/// ]);
+/// </code>
+///
+/// Creating a custom self-test step:
+/// <code>
+/// public class MyCustomSelfTestStep : ISelfTestStep
+/// {
+///     public string Name => "My Custom Test";
+///
+///     public SelfTestStepResult RunStep()
+///     {
+///         // Your test logic here
+///         if (/* test condition passes */)
+///             return SelfTestStepResult.Pass;
+///
+///         if (/* test condition fails */)
+///             return SelfTestStepResult.Fail;
+///
+///         // Still waiting for test to complete
+///         return SelfTestStepResult.Waiting;
+///     }
+///
+///     public void CleanUp()
+///     {
+///         // Clean up any resources used by the test
+///     }
+/// }
+/// </code>
+/// </example>
+public interface ISelfTestRegistry
+{
+    /// <summary>
+    /// Registers the self-test steps for this plugin.
+    /// </summary>
+    /// <param name="steps">The test steps to register.</param>
+    public void RegisterTestSteps(IEnumerable<ISelfTestStep> steps);
+}