libbtrfsutil: python: increase test image size to 4GiB

Some of the subvolume tests fail with ENOSPC with the image size 1G.
Change it to 4G, as 2G is still not enough. Also reset the image
completely before truncating it to the size so there are no stale data.

[ci skip]

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

Author: kdave

libbtrfsutil/python/tests/__init__.py

@@ -65,7 +65,8 @@ def mount_btrfs(self):
         mountpoint = tempfile.mkdtemp()
         try:
             with tempfile.NamedTemporaryFile(delete=False) as f:
-                os.truncate(f.fileno(), 1024 * 1024 * 1024)
+                os.truncate(f.fileno(), 0)
+                os.truncate(f.fileno(), 4 * 1024 * 1024 * 1024)
                 image = f.name
         except Exception as e:
             os.rmdir(mountpoint)