selftests/bpf: fix return value comparison for tests in test_libbpf.sh

The return value for each test in test_libbpf.sh is compared with

    if (( $? == 0 )) ; then ...

This works well with bash, but not with dash, that /bin/sh is aliased to
on some systems (such as Ubuntu).

Let's replace this comparison by something that works on both shells.

Signed-off-by: Quentin Monnet <quentin.monnet@netronome.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>