Add test for 'throttle-ui-updates' config

[See original issue on GitLab](https://gitlab.com/BuildStream/buildstream/-/issues/1250)
In GitLab by [[Gitlab user @tpollard]](https://gitlab.com/tpollard) on Jan 15, 2020, 12:48

## Background

With https://gitlab.com/BuildStream/buildstream/merge_requests/1745 landing, it would probably be beneficial to add tests to check the non restricted behaviour & also ensuring the default tick driven UI is functional. This could potentially harness something along the lines of the pexpect fixtures we already have, parsing the inflight session pager although input interactivity isn't actually needed. Something as simple as checking the status area would probably suffice.

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Locate the existing pexpect fixtures and the code handling the `throttle-ui-updates` configuration, then review how the inflight session pager exposes its status area. Run the relevant existing UI tests and add coverage for unrestricted behavior and the default tick-driven UI, with the status area providing the observable completion check.

Written by the indexing model from the issue text.