Add new unit test for stringfields.

(Copied from reviewboard)
Tests the following:
1. Basic allocation and setting of string fields.
2. Shrinking a string field and re-expanding it.
3. Growing the last allocation in a string field pool.
4. Setting a string to a large value such that a new string field pool must be
allocated.
In each part, we make sure that the string field is accurate (has the correct
value in it), make sure that the 2 bytes before the string field has the correct
capacity for the field, and for tests 2-4, we make sure that the string field is
where we expect it to be in memory.

Also tested:
5. Shrinking a string field and partially re-expanding it.
6. Setting strings in such a way as to create three separate string field pools
and then removing the middle pool.

There is a bug fix in the init function, which ensures the embedded_pool is set
to NULL which is important for stack allocated structures.

Review: https://reviewboard.asterisk.org/r/185/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@251736 65c4cc65-6c06-0410-ace0-fbb531ad65f3