bpf: selftests: Test constant key extraction on irrelevant maps

Test that very high constant map keys are not interpreted as an error
value by the verifier. This would previously fail.

Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
Link: https://lore.kernel.org/r/c0590b62eb9303f389b2f52c0c7e9cf22a358a30.1738689872.git.dxu@dxuuu.xyz


Signed-off-by: Alexei Starovoitov <ast@kernel.org>