selftests/bpf: add tests for attaching invalid fd

selftests/bpf: add tests for attaching invalid fd

Add test cases for situations where adding the following types of file
descriptors to a cpumap entry should fail:
- Non-BPF file descriptor (expect -EINVAL)
- Nonexistent file descriptor (expect -EBADF)

Also tighten the assertion for the expected error when adding a
non-BPF_XDP_CPUMAP program to a cpumap entry.

Signed-off-by: Kohei Enju <enjuk@amazon.com>
Reviewed-by: Toke Høiland-Jørgensen <toke@redhat.com>
Link: https://lore.kernel.org/r/20251208131449.73036-3-enjuk@amazon.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>