selftests/bpf: Convert test_global_funcs test to test_loader framework

Convert 17 test_global_funcs subtests into test_loader framework for
easier maintenance and more declarative way to define expected
failures/successes.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Stanislav Fomichev <sdf@google.com>
Link: https://lore.kernel.org/bpf/20230216045954.3002473-3-andrii@kernel.org