api: Increase /var/image_file size

AUM api test failed due to `No space left on device` error when copying
HMI encrypted bundle to `/var/image_file`.

Signed-off-by: Frédéric Danis <frederic.danis@collabora.com>

run-test-api.sh

@@ -46,7 +46,7 @@ phase_update_mount_detect()
 	local DELTAFILE=$(get_static_delta "" "" ".enc")
 
 	# Create a big enough image file
-	mkfs.ext4 /var/image_file 300000
+	mkfs.ext4 /var/image_file 400000
 
 	local LOOPDEV=$(losetup -Pf --show /var/image_file)