Add the ability to use function calls in `@testset` directly. (#42518)
* Add the ability to use function calls in @testset
This allows for easier factoring of tests into functions, making it possible to `include` a `test/runtests.jl` file without running tests directly and enabling running of specific testsets explicitly.
* Fix doctest of @testset on functions
* Add note about naming of testset for called functions
* Tweak documentation, expand on intended use case
* Add NEWS.md entry
* Fix explicit description behavior, add documentation to docstring
Co-authored-by: Sukera <Seelengrab@users.noreply.github.com>