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/Internal/SelfTestRegistryPluginScoped.cs

@@ -0,0 +1,51 @@
+using System.Collections.Generic;
+
+using Dalamud.IoC;
+using Dalamud.IoC.Internal;
+using Dalamud.Plugin.Internal.Types;
+
+namespace Dalamud.Plugin.SelfTest.Internal;
+
+/// <summary>
+/// Plugin-scoped version of SelfTestRegistry.
+/// </summary>
+[PluginInterface]
+[ServiceManager.ScopedService]
+[ResolveVia<ISelfTestRegistry>]
+internal class SelfTestRegistryPluginScoped : ISelfTestRegistry, IInternalDisposableService
+{
+    [ServiceManager.ServiceDependency]
+    private readonly SelfTestRegistry selfTestRegistry = Service<SelfTestRegistry>.Get();
+
+    private readonly LocalPlugin plugin;
+
+    /// <summary>
+    /// Initializes a new instance of the <see cref="SelfTestRegistryPluginScoped"/> class.
+    /// </summary>
+    /// <param name="plugin">The plugin this service belongs to.</param>
+    [ServiceManager.ServiceConstructor]
+    public SelfTestRegistryPluginScoped(LocalPlugin plugin)
+    {
+        this.plugin = plugin;
+    }
+
+    /// <summary>
+    /// Gets the plugin name.
+    /// </summary>
+    public string PluginName { get; private set; }
+
+    /// <summary>
+    /// Registers test steps for this plugin.
+    /// </summary>
+    /// <param name="steps">The test steps to register.</param>
+    public void RegisterTestSteps(IEnumerable<ISelfTestStep> steps)
+    {
+        this.selfTestRegistry.RegisterPluginSelfTestSteps(this.plugin, steps);
+    }
+
+    /// <inheritdoc/>
+    void IInternalDisposableService.DisposeService()
+    {
+        this.selfTestRegistry.UnregisterPluginSelfTestSteps(this.plugin);
+    }
+}