tests: Add a "pull-test2" that uses slightly more realistic content

tests: Add a "pull-test2" that uses slightly more realistic content

This one actually exercises the delta paths more; I pulled the
data from some local F26AH builds.  One thing I noticed while doing
that is that we try bsdiff on xz'd data, but that won't be useful.
https://github.com/projectatomic/rpm-ostree/issues/470
https://bugzilla.redhat.com/show_bug.cgi?id=1367496

Anyways I verified this test fails without the fixup for `pread()`
in https://github.com/ostreedev/ostree/pull/1312

Closes: #1314
Approved by: jlebon