tests/arena-00.c: Allocate varying block sizes, starting with 0

The old test was too lenient, allocating only 1500 byte block.  This new test
will even allocate block that are larger than the arena block size.

Signed-off-by: Morten Hustveit <morten.hustveit@gmail.com>
arena.c: Get rid of some code duplication

When allocating blocks larger than the maximum arena block size, when can use
the newly created ca_arena_add_pointer function.

Signed-off-by: Morten Hustveit <morten.hustveit@gmail.com>