test_type is not well docummented

[SSE4]

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).