It is not clear what you want to test exactly. Are you sure you are not confusing HS and FS/LS? In most devices those resistors are not static, but configurable by the firmware on FS/LS devices. I'm not even sure there is an official way to force FS device into LS mode. There is a fallback from HS to FS, but not for FS to LS.
Also, operation at USB FS is so dead simple, that there is no reason why it would not be reliable.