selftests/bpf: Use the 25th bit in the "invalid BTF_INFO" test

The bit being checked by this test is no longer reserved after
introducing BTF_KIND_FLOAT, so use the next one instead.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20210226202256.116518-6-iii@linux.ibm.com