tests: Add setup for more realistic repo, change pull-many to use

tests: Add setup for more realistic repo, change pull-many to use

As OSTree has evolved over time, the tests grew with it.  We
didn't start out with static deltas or a summary file, and the
tests reflect this.

What I really want to do is change more of the pull tests, from
corruption/proxying/mirroring etc. to use this more realistic
repo rather than the tiny one the other test creates.

We start by using some of the code from `test-pull-many.sh`, and change that
test to use `--disable-static-deltas` for pull, since the point of that test is
to *not* test deltas.

Still TODO is investigate changing other tests to use this.

Closes: #658
Approved by: jlebon