tests: fix admin upgrade timestamp file check
tests: fix admin upgrade timestamp file check

In #1055, I tried to be helpful but that didn't work out all too well.
We need to recompute the file path since one is in archive mode and the
other in bare mode.

Closes: #1098
Approved by: cgwalters

GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree