Improve reliability of inspecting status of live tmux testing sessions

[ddbeck]

In the tests, starting a test session and getting the contents of a session are fragile. Both merely hope that some command to tmux (new-session in the former and whatever send-keys preceded capture-pane in the latter) completed in a timely manner. This can (and does) cause tests which rely on those methods to fail inconsistently on consecutive test runs.

Those portions of the testing apparatus should be modified to work on some kind of timeout basis (e.g., poll for some known duration before raising a descriptive exception).