test/bl: improve the page_aligned_appender test.

test/bl: improve the page_aligned_appender test.

The idea is to focus on the interface's parts which
are ciritical for `FileWriter` of BlueFS - the sole
user of `page_aligned_appender`.

This includes:
  * preserving the memory contiguity between appends,
    including `append_zero()`.
  * Proper carry-on behaviour when appending another
    `bufferlist` directly to the instance the appender
    is spawned on.

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