tests: Add 404 tests for dirtree objects too
tests: Add 404 tests for dirtree objects too

I now believe the flatpak issue we were hitting was
https://github.com/ostreedev/ostree/pull/1185
but let's add these tests anyways for more coverage.

Closes: #888
Approved by: jlebon

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