test/bufferlist: ensure rebuild_aligned_size_and_memory() always rebuilds.

test/bufferlist: ensure rebuild_aligned_size_and_memory() always rebuilds.

Before the patch the test case was showing an unreliable behaviour
dependent on the underlying memory allocator. It was because
the bufferlist rebuild can be skipped, resulting in unchanged number
of buffers, if all of them begin at aligned addresses.

The commit fixes that by allocating a 4 KiB-aligned buffer and
offsetting it by a small constant (42) to ensure the memory added
to the bufferlist begins at non-4 KiB address.

Signed-off-by: Radoslaw Zarzynski <rzarzyns@redhat.com>