unit tests for src/common/buffer.{cc,h} constructors

The following static constructors are implemented by opaque classes
defined in buffer.cc ( buffer::raw_char, buffer::raw_posix_aligned
etc. ). Testing the implementation of these classes is done by
variations of the calls to the static constructors.

copy(const char *c, unsigned len);
create(unsigned len);
claim_char(unsigned len, char *buf);
create_malloc(unsigned len);
claim_malloc(unsigned len, char *buf);
create_static(unsigned len, char *buf);
create_page_aligned(unsigned len);

The raw_mmap_pages class cannot be tested because it is commented out in
raw_posix_aligned. The raw_hack_aligned class is only tested under Cygwin.
The raw_posix_aligned class is not tested under Cygwin.

The unittest_bufferlist.sh script calls unittest_bufferlist with the
CEPH_BUFFER_TRACK=true environment variable to enable the code
tracking the memory usage. It cannot be done within the bufferlist.cc
file itself because it relies on the initialization of a global
variable  ( buffer_track_alloc ).

When raw_posix_aligned is called on DARWIN, the data is not aligned
on CEPH_PAGE_SIZE because it calls valloc(size) which is the equivalent of
memalign(sysconf(_SC_PAGESIZE),size) and not memalign(CEPH_PAGE_SIZE,size).
For this reason the alignment test is de-activated on DARWIN.

tracker.ceph.com/issues/4066 refs #4066

Signed-off-by: Loic Dachary <loic@dachary.org>