mutation_test: use the correct preferred_max_contiguous_allocation in measuring_allocator
mutation_test: use the correct preferred_max_contiguous_allocation in measuring_allocator

measuring_allocator is a wrapper around standard_allocator, but it exposed
the default preferred_max_contiguous_allocation, not the one from
standard_allocator. Thus managed_bytes allocated in those two allocators
had fragments of different size, and their total memory usage differed,
causing test_external_memory_usage to fail if
standard_allocator::preferred_max_contiguous_allocation was changed from the
default. Fix that.

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla