selftests/bpf: Add a test for ids=0 to verifier_scalar_ids test

selftests/bpf: Add a test for ids=0 to verifier_scalar_ids test

Test that two registers with their id=0 (unlinked) in the cached state
can be mapped to a single id (linked) in the current state.

Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
Link: https://lore.kernel.org/r/20260203165102.2302462-6-puranjay@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>