tests: Split macOS stat() mocking logic

tests: Split macOS stat() mocking logic

On macOS, most of the symbols and declarations that we look at
to determine which versions of stat() we need to mock are not
present; on the other hand, there are some specific wrinkles
that are introduced with Apple Silicon which we will need to
take care of.

To avoid making the logic even more of an opaque mess than it
currently is, move the macOS part to a separate branch.

This commit is better viewed with 'git show -w'.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>