Refactor utils used for tests

[juleswg23]

Refactor assert_rendered*() util functions to only apply to tests, as many are defined identically across multiple files.

https://github.com/posit-dev/great-tables/blob/0b4c5ca3954706314cb34f2fff36cf6085008d1b/tests/test_utils_render_html.py#L14-L39

A secondary question are functions used in both test asserts as well as within the package. These _assert_*() functions are also used in tests, but they are referenced in the form of type checks within the package itself.

https://github.com/posit-dev/great-tables/blob/0b4c5ca3954706314cb34f2fff36cf6085008d1b/great_tables/_utils.py#L62-L86