bpf/tests: Minor restructuring of ALU tests

This patch moves the ALU LSH/RSH/ARSH reference computations into the
common reference value function. Also fix typo in constants so they
now have the intended values.

Signed-off-by: Johan Almbladh <johan.almbladh@anyfinetworks.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20211001130348.3670534-7-johan.almbladh@anyfinetworks.com