Bump to a newer version of ui_test (#306)

We use an outdated version of `ui_test`. This PR updates it to the
latest version (v0.29.1). The changelog is
[here](https://github.com/oli-obk/ui_test/blob/ef35ab91415ffefc1288902f4d679667e460a03d/CHANGELOG.md),
although it appears no changes need to be made on our side.

Author: BD103