Added test for vls-persist

This MR tries to increase coverage of vls-persist to 85%

Filename                                     Regions    Missed Regions     Cover   Functions  Missed Functions  Executed       Lines      Missed Lines     Cover    Branches   Missed Branches     Cover
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vls-persist/src/backup_persister.rs             1013                56    94.47%          51                 5    90.20%         507                29    94.28%           0                 0         -
vls-persist/src/kvv.rs                           445                76    82.92%          42                 9    78.57%         248                31    87.50%           0                 0         -
vls-persist/src/kvv/cloud.rs                     291                19    93.47%          24                 0   100.00%         140                 8    94.29%           0                 0         -
vls-persist/src/kvv/memory.rs                    167                10    94.01%          14                 0   100.00%          90                 6    93.33%           0                 0         -
vls-persist/src/kvv/redb.rs                     1160               108    90.69%          54                18    66.67%         498                52    89.56%           0                 0         -
vls-persist/src/model.rs                         168                16    90.48%          20                 3    85.00%         120                 6    95.00%           0                 0         -
vls-persist/src/util.rs                           52                14    73.08%           4                 1    75.00%          26                 7    73.08%           0                 0         -