Btrfs: separate out tests into their own directory

The plan is to have a bunch of unit tests that run when btrfs is loaded when you
build with the appropriate config option.  My ultimate goal is to have a test
for every non-static function we have, but at first I'm going to focus on the
things that cause us the most problems.  To start out with this just adds a
tests/ directory and moves the existing free space cache tests into that
directory and sets up all of the infrastructure.  Thanks,

Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>