selftests/bpf: Add tests for user- and non-CO-RE BPF_CORE_READ() variants

Add selftests validating that newly added variations of BPF_CORE_READ(), for
use with user-space addresses and for non-CO-RE reads, work as expected.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20201218235614.2284956-4-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>