storagenode/reputation: unit test coverage

[neo-cypher]

This change implements unit tests for service.go in the reputation package. This change increases test coverage by 3.6%.

What:

Why:

Please describe the tests:

• Test 1:
• Test 2:

Please describe the performance impact:

Code Review Checklist (to be filled out by reviewer)

• NEW: Are there any Satellite database migrations? Are they forwards and backwards compatible?
• Does the PR describe what changes are being made?
• Does the PR describe why the changes are being made?
• Does the code follow our style guide?
• Does the code follow our testing guide?
• Is the PR appropriately sized? (If it could be broken into smaller PRs it should be)
• Does the new code have enough tests? (every PR should have tests or justification otherwise. Bug-fix PRs especially)
• Does the new code have enough documentation that answers "how do I use it?" and "what does it do?"? (both source documentation and higher level, diagrams?)
• Does any documentation need updating?
• Do the database access patterns make sense?