selftests/bpf: Close fd on unexpected success in signed loader

The signature_enforced, signature_too_large, signature_zero_size and
signature_bad_keyring subtests load a program that must be rejected,
but leave the fd open if the kernel unexpectedly accepts the load:
test_progs asserts record the failure and continue, so the fd would
linger for the rest of the run. Just close it.

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20260708184107.369182-1-daniel@iogearbox.net
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>