commitlog_test: Modify test_allocation_failure to not require huge segment
commitlog_test: Modify test_allocation_failure to not require huge segment

Explicitly use up all the memory in the system as best as we can instead
Still not super reliable, but should have less side effects. And work better
with pre-allocated segment files

duarten/scylla,avikivity/scylla,duarten/scylla,duarten/scylla,raphaelsc/scylla,scylladb/scylla,kjniemi/scylla,scylladb/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla,raphaelsc/scylla,scylladb/scylla,kjniemi/scylla,kjniemi/scylla,raphaelsc/scylla