selftests/bpf: convert bpf_rdonly_cast() uses to bpf_core_cast() macro

Use more ergonomic bpf_core_cast() macro instead of bpf_rdonly_cast() in
selftests code.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20240130212023.183765-3-andrii@kernel.org


Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>