Hi,
That's tough if you already have the requirement in your contract, but they just dont comply. Do you know why they do not test their DR plans?
One measure you could look to add to the contract, is the requirement to allow your firm to participate in an annual DR test, that way you have a little more visibility into what is going on over there, and can have some input into the time table. You may also look to compel them to engage a third party auditor to review their controls if they do not test within the agreed timeframe (at their cost).
But ultimately if you already have the requirement established and they are not acting in good faith or treating you as a valued customer, you would have to ask yourself as firm, where you would sit on their priority list in the event of a real incident ?