selftests/bpf: Refactor out helper functions for a few tests

These helper functions will be used later new tests as well.
There are no functionality change.

Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Reviewed-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20240410043542.3738166-1-yonghong.song@linux.dev


Signed-off-by: Alexei Starovoitov <ast@kernel.org>