Overhaul core Tracker: add tests for `torrent` mod

[josecelano]

Parent issue: #1226

Coverage before:

Filename                                                                    Regions    Missed Regions     Cover   Functions  Missed Functions  Executed       Lines      Missed Lines     Cover    Branches   Missed Branches     Cover
---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
packages/tracker-core/src/torrent/manager.rs                                     11                 5    54.55%           3                 1    66.67%          28                11    60.71%           0                 0         -
packages/tracker-core/src/torrent/repository/in_memory.rs                        49                 5    89.80%          27                 2    92.59%         298                 9    96.98%           0                 0         -
packages/tracker-core/src/torrent/repository/persisted.rs                         3                 0   100.00%           3                 0   100.00%          11                 0   100.00%           0                 0         -
packages/tracker-core/src/torrent/services.rs                                    39                10    74.36%          20                 2    90.00%         323                14    95.67%           0                 0         -