Add more tests

[Ninjani]

Problem

Needs more thorough testing of all functionality and edge-cases

Additional context

Results from cargo llvm-cov test -- --test-threads=1:

Filename                         Regions    Missed Regions     Cover   Functions  Missed Functions  Executed       Lines      Missed Lines     Cover    Branches   Missed Branches     Cover
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
configuration.rs                     556               500    10.07%          70                62    11.43%         800               741     7.38%           0                 0         -
errors.rs                              4                 4     0.00%           2                 2     0.00%           2                 2     0.00%           0                 0         -
gooseberry/cli.rs                    153               130    15.03%          46                35    23.91%          99                87    12.12%           0                 0         -
gooseberry/database.rs               204                80    60.78%          24                 2    91.67%         181                22    87.85%           0                 0         -
gooseberry/knowledge_base.rs         314               162    48.41%          41                20    51.22%         373               109    70.78%           0                 0         -
gooseberry/mod.rs                    461               243    47.29%          64                29    54.69%         545               213    60.92%           0                 0         -
gooseberry/search.rs                 168               168     0.00%          25                25     0.00%         311               311     0.00%           0                 0         -
main.rs                                7                 2    71.43%           2                 0   100.00%           7                 0   100.00%           0                 0         -
utils.rs                              51                33    35.29%          13                 7    46.15%          97                57    41.24%           0                 0         -
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
TOTAL                               1918              1322    31.07%         287               182    36.59%        2415              1542    36.15%           0                 0         -