selftests/bpf: Tests for enum fwd resolved as full enum64

A set of test cases to verify enum fwd resolution logic:
- verify that enum fwd can be resolved as full enum64;
- verify that enum64 fwd can be resolved as full enum;
- verify that enum size is considered when enums are compared for
  equivalence.

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20221101235413.1824260-2-eddyz87@gmail.com