selftests/bpf: fix the expected messages

Recent changes added extack to program replacement path,
expect extack instead of generic messages.

Fixes: 01dde20c ("xdp: Provide extack messages when prog attachment failed")
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>