PEP 750: Improve `TemplateIter` tests

[sobolevn]

Bug report

Right now there are some test that ensure that t string can be iterated over.
But, we never test TemplateIter behavior in detail.

I propose to add several test that this type behaves how we expect it to.

Linked PRs

• gh-133213: Add tests for string.templatelib.TemplateIter #133215