selftests/bpf: add Error: prefix in check_extack helper

Currently the test only checks errors, not warnings, so save typing
and prefix the extack messages with "Error:" inside the check helper.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Quentin Monnet <quentin.monnet@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>