tests: Add a big (many objects) pull

tests: Add a big (many objects) pull

This would be more likely to tickle things like
https://github.com/ostreedev/ostree/issues/601
reliably.

Also, while working on the curl backend, I hit on the fact that curl doesn't
queue (by default, you can enable) and will happily create 20000+ concurrent TCP
connections if you try. Having this test would have made that more likely to
fail.

Closes: #650
Approved by: giuseppe