Add automated tests for downloading adblock / tracking protection data files

[bsclifton]

Carried over from brave/browser-laptop#125

The tests should:

• Check for proper handling for 404 if adblock data file isn't uploaded correctly
• Check for proper handling if adblock server is offline
• Check for proper handling if time elapsed since last check is > 1 day
• Check if there is proper handling when reading cached storage from disk
• Check if there is proper handling when a 2nd window is open
• Check for proper handling when etags match
• Check for proper handling when etags do not match