bpf: more tests for mixed signed and unsigned bounds checks

Add a couple of more test cases to BPF selftests that are related
to mixed signed and unsigned checks.

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>