test/bufferlist: assert the rebuild in rebuild_aligned_size_and_memory() actually...

test/bufferlist: assert the rebuild in rebuild_aligned_size_and_memory() actually happens.

For the investigation of failures like the following one:

```
[ RUN      ] BufferList.rebuild_aligned_size_and_memory
../src/test/bufferlist.cc:1865: Failure
Expected equality of these values:
  bl.get_num_buffers()
    Which is: 2
  1
[  FAILED  ] BufferList.rebuild_aligned_size_and_memory (0 ms)
```

The test case assumes the rebuild before the failed clause **always**
happens while `bufferlist::rebuild_aligned_size_and_memory()` skips it
if buffers are already aligned.

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