test_type is not well docummented

from https://github.com/conan-io/conan-center-index/pull/8947#discussion_r789910861

the search results for `test_type` are only 4 pages, where it is briefly mentioned, but there is no reference page for the attribute

possible values (from code) seems to be: None, `explicit`, `requires`, `build_requires` (and seems like it can be None-type, string or list of strings, as I can deduce).