Add support for test explorer

[nojaf]

Following up on #796, I'm trying to address the remaining work.
@baronfel, I changed the parameter type to a new type, but I'm guessing there is a bit more to it.
Could you point me in the right direction on what I need to do with TestDetectedNotification?

[baronfel]

@nojaf what I meant by a new type is that the PlainNotification type is serialized and sent to VSCode (and other clients) as a { Content: string } structure, and I'd much prefer if instead it could be sent with a more useful structure. We already have a useful structure for the message and I would much rather send this structure in the notification message (letting newtonsoft.json serialize it to json) than sending a raw string to the client.

Essentially, IMO we should be reducing the amount of usage of PlainNotification in favor of typed responses all the time, and integrating that into callsites in Ionide as we change that contract.

[nojaf]

@baronfel this first test worked on my machine.
I'm not really familiar with Expecto.
Please let me know if this is more or less what you had in mind.

[baronfel]

These tests are exactly what I had in mind! As we expand detection support we can of course flesh them out 👍

[baronfel]

This is great - the failing windows tests are the flaky ones that are not related to this effort. I'm good to merge this. Once it merges we can look at the Ionide side of things (which will likely need a slight tweak as the shape of the new notification has changed slightly from the original pass of this work).