selftests/bpf: Add test for signed 32-bit bound check bug

After a 32-bit load followed by a branch, the verifier would reduce the
maximum bound of the register to 0x7fffffff, allowing a user to bypass
bound checks. Ensure such a program is rejected.

In the second test, the 64-bit compare should not sufficient to
determine whether the signed 32-bit lower bound is 0, so the verifier
should reject the second branch.

Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>