selftests/bpf: Remove all the uses of deprecated bpf_prog_load_xattr()

Migrate all the selftests that were still using bpf_prog_load_xattr().
Few are converted to skeleton, others will use bpf_object__open_file()
API.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20211201232824.3166325-7-andrii@kernel.org