tests: make alloc_test work with many --memory sizes
tests: make alloc_test work with many --memory sizes

Account for internal overhead used by the seastar allocator.

jonathanleang/seastar,acbellini/seastar,dwdm/seastar,avikivity/seastar,avikivity/seastar,avikivity/seastar,flashbuckets/seastar,xtao/seastar,syuu1228/seastar,anzihenry/seastar,dreamsxin/seastar,linearregression/seastar,scylladb/scylla-seastar,jonathanleang/seastar,joerg84/seastar,acbellini/seastar,koolhazz/seastar,syuu1228/seastar,bowlofstew/seastar,leejir/seastar,acbellini/seastar,cloudius-systems/seastar,scylladb/seastar,printedheart/seastar,jonathanleang/seastar,scylladb/seastar,xtao/seastar,shyamalschandra/seastar,wildinto/seastar,bowlofstew/seastar,sjperkins/seastar,syuu1228/seastar,cloudius-systems/seastar,hongliangzhao/seastar,norcimo5/seastar,koolhazz/seastar,scylladb/scylla-seastar,norcimo5/seastar,linearregression/seastar,printedheart/seastar,printedheart/seastar,hongliangzhao/seastar,stamhe/seastar,kjniemi/seastar,chunshengster/seastar,kjniemi/seastar,tempbottle/seastar,scylladb/seastar,kjniemi/seastar,chunshengster/seastar,joerg84/seastar,shyamalschandra/seastar,stamhe/seastar,hongliangzhao/seastar,flashbuckets/seastar,scylladb/scylla-seastar,dreamsxin/seastar,flashbuckets/seastar,dwdm/seastar,tempbottle/seastar,leejir/seastar,sjperkins/seastar,bowlofstew/seastar,cloudius-systems/seastar,tempbottle/seastar,sjperkins/seastar,dwdm/seastar,wildinto/seastar,dreamsxin/seastar,anzihenry/seastar,xtao/seastar,leejir/seastar,chunshengster/seastar,wildinto/seastar,koolhazz/seastar,linearregression/seastar,shyamalschandra/seastar,stamhe/seastar,joerg84/seastar,norcimo5/seastar,anzihenry/seastar