increase test coverage for channel, cycle, drop & zip

[jlc-christie]

Please provide a brief description of the change.

Due to a change in how pull iterators are implemented, coverage dropped on these tests.

Which issue does this change relate to?

Closes #115

Contribution checklist.

Replace the space in each box with "X" to check it off.

• [X] I have read and understood the CONTRIBUTING guidelines
• [X] My code is formatted (make check)
• [X] I have run tests (make test)
• [X] All commits in my PR conform to the commit hygiene section
• [X] I have added relevant tests
• [X] I have not added any dependencies

Additional context

N/A

[codecov[bot]]

Codecov Report

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

Project coverage is 100.00%. Comparing base (b35ab35) to head (f23adea). Report is 1 commits behind head on v2.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## v2 #122 +/- ## =========================================== + Coverage 95.03% 100.00% +4.96% =========================================== Files 16 16 Lines 322 322 =========================================== + Hits 306 322 +16 + Misses 8 0 -8 + Partials 8 0 -8 ```

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