refactor(tests): improve x/reports tests using gomock

[dadamu]

Description

This PR improves the x/reports tests using gomock.

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Author Checklist

All items are required. Please add a note to the item if the item is not applicable and please add links to any relevant follow up issues.

I have...

• [x] included the correct type prefix in the PR title
• [ ] added ! to the type prefix if API or client breaking change
• [x] targeted the correct branch (see PR Targeting)
• [ ] provided a link to the relevant issue or specification
• [ ] followed the guidelines for building modules
• [ ] included the necessary unit and integration tests
• [ ] added a changelog entry to CHANGELOG.md
• [ ] included comments for documenting Go code
• [ ] updated the relevant documentation or specification
• [ ] reviewed "Files changed" and left comments if necessary
• [ ] confirmed all CI checks have passed

Reviewers Checklist

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I have...

• [ ] confirmed the correct type prefix in the PR title
• [ ] confirmed ! in the type prefix if API or client breaking change
• [ ] confirmed all author checklist items have been addressed
• [ ] reviewed state machine logic
• [ ] reviewed API design and naming
• [ ] reviewed documentation is accurate
• [ ] reviewed tests and test coverage
• [ ] manually tested (if applicable)

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (61393a0) 80.79% compared to head (59bc4f3) 80.80%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1184 +/- ## ======================================= Coverage 80.79% 80.80% ======================================= Files 194 194 Lines 17310 17310 ======================================= + Hits 13986 13987 +1 Misses 2727 2727 + Partials 597 596 -1 ``` | [Impacted Files](https://app.codecov.io/gh/desmos-labs/desmos/pull/1184?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=desmos-labs) | Coverage Δ | | |---|---|---| | [x/reports/keeper/external\_hooks.go](https://app.codecov.io/gh/desmos-labs/desmos/pull/1184?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=desmos-labs#diff-eC9yZXBvcnRzL2tlZXBlci9leHRlcm5hbF9ob29rcy5nbw==) | `68.88% <100.00%> (+2.22%)` | :arrow_up: |

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