selftests/bpf: Set BPF program flags non-destructively

A couple of the BPF selftests would set the program flags without
looking at the existing program flags, overriding any other flag values.
To avoid issues when we change libbpf to carry section flags in
prog_flags, change this to always set the flag value non-destructively
by OR'ing it with the existing flags.

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-v3-2-4eb6f14968b0@redhat.com