selftests: bpf: add zero extend checks for ALU32 and/or/xor

Add three tests to test_verifier/basic_instr that make sure that the
high 32-bits of the destination register is cleared after an ALU32
and/or/xor.

Signed-off-by: Björn Töpel <bjorn.topel@gmail.com>
Acked-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>