selftests: openat2: Print also errno in failure messages

In E_func() macro, on error, print also errno in order to aid debugging.

Cc: Aleksa Sarai <cyphar@cyphar.com>
Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>