selftests/bpf: Add ret_mod multi func test

Adding extra test to existing modify_return test to
test this with multi func program attached on top
of the modify return program.

Because the supported wildcards do not allow us to
match both bpf_fentry_test* and bpf_modify_return_test,
adding extra code to look it up in kernel's BTF.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>