Gregor Zattler observed that tags could be unintentionally hidden in
the "All tags" view, and Tomi Ollia worked out [2] that the issue was tags
that only occured on excluded messages. This test reproduces that bug.
[1]: id:87wox1vovj.fsf@len.workgroup
[2]: id:m28t9faaim.fsf@guru.guru-group.fi