selftests/bpf: Fix attach_probe in parallel mode

This patch makes attach_probe uses its own method as attach point,
avoiding conflict with other tests like bpf_cookie.

Signed-off-by: Yucong Sun <sunyucong@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20211025223345.2136168-4-fallentree@fb.com