TEST-65-ANALYZE: only mount /usr if necessary
Follow-up for
8f764b32eabd3d39abfc0f4eaffd282223df4768.
Otherwise, we fail to umount several directories on shutdown.
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[ 15.684430] systemd[1]: Failed unmounting tmp-root-proc.mount.
[ 15.689379] systemd[1]: Failed unmounting tmp-root-run.mount.
[ 15.710572] systemd[1]: Failed unmounting tmp-root-sys-fs-cgroup.mount.
[ 15.913953] systemd[1]: Failed unmounting tmp-root-dev.mount.
[ 15.924603] systemd[1]: Failed unmounting tmp-root-sys.mount.
[ 15.938207] systemd[1]: Failed unmounting tmp-root.mount.
[ 15.952395] systemd[1]: Failed unmounting tmp.mount.