Fix/distribution allow slash

[neolynx]

Fixes #115

Sperrseeods https://github.com/aptly-dev/aptly/pull/1089

Requirements

All new code should be covered with tests, documentation should be updated. CI should pass.

Description of the Change

Add tests for publishing a mirror with slash (/) in the distribution: buster/updates

Checklist

• [ ] unit-test added (if change is algorithm)
• [x] functional test added/updated (if change is functional)
• [ ] man page updated (if applicable)
• [ ] bash completion updated (if applicable)
• [ ] documentation updated
• [ ] author name in AUTHORS

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 74.70%. Comparing base (32595e7) to head (cb3d616).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1269 +/- ## ========================================== - Coverage 74.70% 74.70% -0.01% ========================================== Files 144 144 Lines 16221 16216 -5 ========================================== - Hits 12118 12114 -4 + Misses 3159 3158 -1 Partials 944 944 ```

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