Consume new rich test events in new Test Explorer UI

[dbartol]

This PR updates the new test UI to consume the richer events emitted by codeql test run starting in CLI 2.13.1. We now get a notification when a test starts executing, so we can update the Test Explorer with the right spinning icon.

I had to do a little work in the CLI server client code to handle multiple streaming output formats.

I refactored some of the tracking of which tests are in which directory into a new TestRunContext class.

The legacy test adapter code has been updated to consume the new format for the existing TestCompleted event, but has not been updated to consume the new events.