backups: Increase unit testing coverage of backups module

Copied from original issue #433 (closed).

Current coverage:

plinth/modules/backups/__init__.py                             122     82     16      0    28.99%
plinth/modules/backups/api.py                                  192     45     60     11    74.60%
plinth/modules/backups/components.py                            58      0     26      0   100.00%
plinth/modules/backups/decorators.py                            14     14      4      0     0.00%
plinth/modules/backups/errors.py                                 6      0      0      0   100.00%
plinth/modules/backups/forms.py                                132     61     36      1    46.43%
plinth/modules/backups/manifest.py                               2      2      0      0     0.00%
plinth/modules/backups/repository.py                           284    182     58      0    29.82%
plinth/modules/backups/schedule.py                             142      4     56      2    96.97%
plinth/modules/backups/store.py                                 32      1     12      1    90.91%
plinth/modules/backups/urls.py                                   4      4      0      0     0.00%
plinth/modules/backups/views.py                                303    303     42      0     0.00%