selftests/bpf: switch tests to new bpf_object__open_{file, mem}() APIs

Verify new bpf_object__open_mem() and bpf_object__open_file() APIs work
as expected by switching test_attach_probe test to use embedded BPF
object and bpf_object__open_mem() and test_reference_tracking to
bpf_object__open_file().

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>