Test switching from unclutter to hhp in the test suite

The hope is that hhp will not trigger the issues we have with unclutter, described in #8928 (closed). hhp is part of xmonad-utils, which unfortunately isn’t in Debian, and there’s also a small C implementation.

Parent Task: #8928 (closed)

Original created by @anonym on 9009 (Redmine)