selftests/bpf: add selftest that use test_libbpf_open

This script test_libbpf.sh will be part of the 'make run_tests'
invocation, but can also be invoked manually in this directory,
and a verbose mode can be enabled via setting the environment
variable $VERBOSE like:

 $ VERBOSE=yes ./test_libbpf.sh

The script contains some tests that are commented out, as they
currently fail.  They are reminders about what we need to improve
for the libbpf loader library.

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>