tools/bpf: add bpffs percpu map pretty print tests in test_btf

The bpf selftest test_btf is extended to test bpffs
percpu map pretty print for percpu array, percpu hash and
percpu lru hash.

Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>