btrfs: tests, use BTRFS_FS_STATE_DUMMY_FS_INFO instead of dummy root

Now that we have a dummy fs_info associated with each test that
uses a root, we don't need the DUMMY_ROOT bit anymore.  This lets
us make choices without needing an actual root like in e.g.
btrfs_find_create_tree_block.

Signed-off-by: Jeff Mahoney <jeffm@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>