x/mint: refactor AfterEpochEnd hook and add more tests

[p0mvn]

Background

This is a follow-up to #1839

After the refactor of DistributeMintCoin and the refactor of hook tests are complete, we can investigate whether AfterEpochEnd Hook can be

Acceptance Criteria

• Make sure #1916 and #1918 are complete
• Investigate if AfterEpochEnd can further be refactored and unit tested
• follow-up issues created if applicable

[p0mvn]

Blocked by #1916 and #1918