Parent issue: #1181

After extracting the new package I want to review the tests. We had a lot of high-level tests because It was very hard to unit test the old core::Tracker. Now it should be easier to write unit tests for the package.

Sub-tasks

• Overhaul core Tracker: add tests for authentication mod #1231
• Overhaul core Tracker: add tests for whitelist mod #1235
• Overhaul core Tracker: add tests for torrent mod #1240
• Overhaul core Tracker: add tests for announce_handler mod #1247
• Overhaul core Tracker: add tests for scrape_handler mod #1249
• Overhaul core Tracker: add tests for error mod #1250
• Overhaul core Tracker: add tests for databases mod #1251