lib/tests: randstruct: Add deep function pointer layout test

The recent fix in commit c2ea09b193d2 ("randstruct: gcc-plugin: Remove
bogus void member") has fixed another issue: it was not always detecting
composite structures made only of function pointers and structures of
function pointers. Add a test for this case, and break out the layout
tests since this issue is actually a problem for Clang as well[1].

Link: https://github.com/llvm/llvm-project/issues/138355 [1]
Link: https://lore.kernel.org/r/20250502224116.work.591-kees@kernel.org


Signed-off-by: Kees Cook <kees@kernel.org>