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.
