CodeHintManager unit tests

[iwehrman]

Previously, commit 5878608 broke code hint provider selection (later fixed by #3208). This went unnoticed for two weeks because there are no unit tests for the CodeHintManager.

There should be CodeHintManager unit tests for:

1. provider registration;
2. provider selection by language and priority, including tests for the special "all" language mode; and
3. session creation and destruction.

[pthiess]

Reviewed - @RaymondLim , Medium Priority - we'd want to improve that before we touch the CodeHintMgr. again.

[RaymondLim]

Reassign it to @peterflynn

[RaymondLim]

Unit tests reviewed and ran.

@iwehrman I'm closing it since the new test cases with 2 old test cases cover all you've listed, but you can always double-check if you'd like.