selftests/bpf: Add test validating failure on ambiguous relocation value

Add test simulating ambiguous field size relocation, while fields themselves
are at the exact same offset.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200818223921.2911963-5-andriin@fb.com