TST: coverage for skipped tests in io/formats/test_to_html.py
- [ N/A] precursor to #22786
- [ ] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ N/A] whatsnew entry

There are currently 3 skipped tests in `io/formats/test_to_html.py`; `test_to_html_truncate`, `test_to_html_truncate_multi_index` and `test_to_html_truncate_multi_index_sparse_off`

Only `test_to_html_truncate` is required to provide regression coverage for #22786

However, it is probably appropriate for any changes to `test_to_html_truncate` to also be applied to the other two skipped tests within the scope of this PR.

cc @jreback 