selftests/bpf: Refactor some functions for kprobe_multi_test

Refactor some functions in kprobe_multi_test.c to extract
some helper functions who will be used in later patches
to avoid code duplication.

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20240326041503.1198982-1-yonghong.song@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>