btrfs: add self test for bytes_index free space cache

I noticed a few corner cases when looking at my bytes_index patch for
obvious bugs, so add a bunch of tests to validate proper behavior of the
bytes_index tree.  A couple of basic tests to make sure it puts things
in the correct order, and then more complicated tests to make sure it
re-arranges bitmap entries properly and does the right thing when we try
to make allocations.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: David Sterba <dsterba@suse.com>