Otherwise it remains there, and another test case accidentally
uses it on refresh, which then makes another later test fail,
as the hierarchy is already merged:
[ 203.969708] TEST-50-DISSECT.sh[890]: + systemd-sysext status
[ 203.981831] TEST-50-DISSECT.sh[2795]: HIERARCHY EXTENSIONS SINCE
[ 203.982196] TEST-50-DISSECT.sh[2795]: /opt app0 Mon 2025-09-08 11:49:11 UTC
[ 203.982551] TEST-50-DISSECT.sh[2795]: /usr app0 Mon 2025-09-08 11:49:11 UTC
[ 204.119772] TEST-50-DISSECT.sh[2799]: Hierarchy '/usr' is already merged.
Fixes https://github.com/systemd/systemd/issues/38282