✅ Add core tests to improve coverage

[JMicheli]

This pull request adds additional tests to the core crate. So far I have added:

• utils tests
• filesystem/common tests
• filesystem/archive tests

[codecov]

Codecov Report

Attention: Patch coverage is 93.61702% with 6 lines in your changes missing coverage. Please review.

Files	Coverage Δ
core/src/config/mod.rs	11.76% <ø> (ø)
core/src/filesystem/archive.rs	97.77% <100.00%> (ø)
core/src/filesystem/common.rs	51.57% <100.00%> (ø)
core/src/utils.rs	100.00% <100.00%> (ø)
core/src/db/entity/notifier.rs	0.00% <0.00%> (ø)
core/src/db/entity/emailer/entity.rs	0.00% <0.00%> (ø)

[aaronleopold]

LGTM! Thank you!