test: remove extension from /var/lib/extensions at the end of the test

test: remove extension from /var/lib/extensions at the end of the test

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

(cherry picked from commit 9cc9e14ed4671eb8c0424b465504202371e17419)