tests: weaken the assertion in test_toplevel().

nosetests will not prepend dnf_toplevel() on the PYTHONPATH if it's already
there.
repo: do not try to download or erase packages from a local repo. (RhBug:965114)
update LibrepoCallbackAdaptor. (RhBug:963627)

- use _total instead of _running.
- initialize it in begin(), not in __init__().
- when total_to_download changes, re-start() the progress.
Merge branch 'master' of github.com:zde/dnf