Closes#795
- Added structure `SaplingParamsSourceURL` which holds source URLs for
sapling-spend file and sapling-output file. URLs are ultimately passed
to `SaplingParameterDownloader`.
- This is done so it's possible to pass different URLs when running
tests.
- sapling-spend file and sapling-output file are in tests bundle. And
when running tests these files are loaded from bundle and not from
network.
Michal Fousek