fix: submitting with hint when only main certifier is configured

[poszu]

Closes #523

The Registration now always adds the main key to the map of trusted keys to fix registering with a hint when the trusted keys dir is not configured.

I added tests to cover this scenario and refactored the tests to reduce boilerplate, obscuring the test logic.

[codecov[bot]]

Codecov Report

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

Project coverage is 80.8%. Comparing base (5591b98) to head (66d3188). Report is 2 commits behind head on develop.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #525 +/- ## ========================================= + Coverage 80.6% 80.8% +0.2% ========================================= Files 27 27 Lines 1868 1871 +3 ========================================= + Hits 1506 1513 +7 + Misses 231 228 -3 + Partials 131 130 -1 ```

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