selftest: add tests for O_EMPTYPATH

selftest: add tests for O_EMPTYPATH

Add tests for the new O_EMPTYPATH flag of openat(2)/openat2(2).

Also, the current openat2 tests include a helper header file that
defines the necessary structs and constants to use openat2(2), such as
struct open_how. This may result in conflicting definitions when the
system header openat2.h is present as well.

So add openat2.h generated by 'make headers' to the uapi header
files in ./tools/include and remove the helper file definitions of
the current openat2 selftests.

Signed-off-by: Jori Koolstra <jkoolstra@xs4all.nl>
Link: https://patch.msgid.link/20260424114611.1678641-3-jkoolstra@xs4all.nl
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>