selftests/bpf: Add test involving restrict type qualifier

This change adds a type based test involving the restrict type qualifier
to the BPF selftests. On the btfgen path, this will verify that bpftool
correctly handles the corresponding RESTRICT BTF kind.

Signed-off-by: Daniel Müller <deso@posteo.net>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Quentin Monnet <quentin@isovalent.com>
Link: https://lore.kernel.org/bpf/20220706212855.1700615-3-deso@posteo.net