Add reset() function with proper tests for `GroupByHashMapOptimization`

[RobinTF]

This commit is a preparation for the lazy GroupBy. It adds several missing unit tests, and adds a small function to several classes in the GroupBy module.

[codecov[bot]]

Codecov Report

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

Project coverage is 94.16%. Comparing base (dc71166) to head (a9abbc6). Report is 3 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1499 +/- ## ======================================= Coverage 94.15% 94.16% ======================================= Files 347 347 Lines 25627 25641 +14 Branches 3445 3450 +5 ======================================= + Hits 24130 24145 +15 + Misses 1455 1454 -1 Partials 42 42 ```

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[sonarcloud[bot]]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud