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 <[e-mail]>
Reviewed-by: Jakub Kicinski <[e-mail]>
Signed-off-by: Alexei Starovoitov <[e-mail]>

tools/testing/selftests/bpf/test_libbpf.sh

···
 # Determine selftest success via shell exit code
 exit_handler()
 {
-	if (( $? == 0 )); then
+	if [ $? -eq 0 ]; then
 		echo "selftests: $TESTNAME [PASS]";
 	else
 		echo "$TESTNAME: failed at file $LAST_LOADED" 1>&2