selftests/bpf: Add nested trust selftests suite

Now that defining trusted fields in a struct is supported, we should add
selftests to verify the behavior. This patch adds a few such testcases.

Signed-off-by: David Vernet <void@manifault.com>
Link: https://lore.kernel.org/r/20230125143816.721952-4-void@manifault.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>