Allow tests to identify the TestPlatform they are being run on.

[DrMarcII]

I am working on an in-browser/webdriver hybrid testing method. This requires exposing a WebDriver API from the test server for each browser. From within the test, I need to know which of those WebDriver sessions to connect to (if running with multiple platforms simultaneously).

My current idea is to make the session id match up with the platform id. Then if I could grab the platform id in the test, I could create the correct webdriver client object.

[nex3]

This is blocked on #48. This is the sort of information that should be exposed somewhere on the Invoker API.