More hardening of the completion test

[alexgav]

We now wait for a specific item in the actual shown completion UI, and also wait for the correct text in the editor post-commit to account for actions such as OnAutoInsert

1. We now actually open completion UI. Previously we may or may not have had the UI open, because IAsyncCompletionBroker.TriggerCompletion doesn't actually cause completion pop-up window UI to show (at least per doc comments). IAsyncCompletionSession.OpenOrUpdate actually shows completion window UI.
2. We now wait for a specified item to be present and selected prior to trying to commit. We will re-update (re-calculate) completion items while waiting.
3. We now wait for the correct text to be in the editor. After completion commit, there may be another action, such as OnAutoInsert that may modify the text further. We specify the final expected state of the text, so we should wait for it to be fully updated.