selftests/bpf: Test skipping stacktrace

Add a test case for stacktrace with skip > 0 using a small sized
buffer. It didn't support skipping entries greater than or equal to
the size of buffer and filled the skipped part with 0.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20220314182042.71025-2-namhyung@kernel.org