bpf: add further test cases around div/mod and others

Update selftests to relfect recent changes and add various new
test cases.

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