selftests/bpf: Check for flag presence in bpf_program__flags()

Add a check that the BPF_F_XDP_HAS_FRAGS and BPF_F_SLEEPABLE flags show
up in bpf_program__flags() when opening a BPF program with the flag
definitions in their section definitions.

Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20260901-libbpf-frags-flags-v3-4-4eb6f14968b0@redhat.com