Improve code coverage for aggregation tests

[AvitalFineRedis]

Pull Request check-list

Please make sure to review and check all of these items:

• Does $ tox pass with this change (including linting)?
• Do the CI tests pass with this change (enable it first in your forked repo and wait for the github action build to finish)?
• Is the new or changed code fully tested?
• Is a documentation update included (if this change modifies existing APIs, or introduces new ones)?

NOTE: these things are not required to open a PR and can be done
afterwards / while the PR is open.

Description of change

Add tests for some aggregation function that hadn't been tested yet.

Two things that were left uncovered (from aggregation.py):

1. verbatim
2. cursor

[codecov-commenter]

Codecov Report

Merging #1713 (0f68afd) into master (3838493) will increase coverage by 0.47%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1713      +/-   ##
==========================================
+ Coverage   89.05%   89.52%   +0.47%     
==========================================
  Files          53       53              
  Lines       11078    11127      +49     
==========================================
+ Hits         9865     9961      +96     
+ Misses       1213     1166      -47     

Impacted Files	Coverage Δ
redis/commands/search/aggregation.py	79.62% <ø> (+26.61%)	⬆️
tests/test_search.py	98.93% <100.00%> (+0.36%)	⬆️
redis/__init__.py	84.61% <0.00%> (-15.39%)	⬇️
tests/conftest.py	93.50% <0.00%> (+1.29%)	⬆️

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