How to install 4-prong dryer in 3-prong receptacle with 4-wire cut ground?
That would be fine, as long as you make sure that unused ground wire is in tact and properly terminated back at the panel.  

Testing continuity to ground is not IMO a sufficient test, since a neutral and a ground would both show continuity to ground.  Hopefully the cable is run exposed where you can see it and verify that it goes where it should in the panel.