selftests/bpf: Refactor out some functions in ns_current_pid_tgid test

Refactor some functions in both user space code and bpf program
as these functions are used by later cgroup/sk_msg tests.
Another change is to mark tp program optional loading as later
patches will use optional loading as well since they have quite
different attachment and testing logic.

There is no functionality change.

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20240315184904.2976123-1-yonghong.song@linux.dev