Write simple test_visualize test suite

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There aren't any smoke tests that check that visualization is working. The end to end test does call h.visualize, but we recently saw that those tests weren't running. (They were marked "slow" and we were skipping slow tests)

Since the end to end tests were covering train, infer, umap and visualize, and because we already have specific unit tests for train, infer and umap, we should just focus on writing a small suite for visualize.

Once the visualize unit tests are in place, we should:
- Remove the end to end tests
- Remove the "not slow" arguments in pre-commit pytest-check, testing-and-coverage.yml, and smoke-test.yml

Start by locating the existing end-to-end tests that call h.visualize and the project's existing unit-test conventions. Add a small visualize test suite, then remove those end-to-end tests and the “not slow” arguments from pre-commit pytest-check, testing-and-coverage.yml, and smoke-test.yml; done means visualization is covered without slow-test exclusions.

Written by the indexing model from the issue text.