Seeing the long-term affects of awarding partial credit has convinced me the product owner and ScrumMaster should not award credit for something that isn't done per the agreement.  An exception might be if the team came to you well before the review meeting to renegotiate, but even then the outcome should be reduced scope (fewer features) rather than reduced quality (fewer tests).

The times it's hardest to do this are the times you have the most to gain from doing this.  Awarding credit for work not done to the agreement robs the team of growth opportunities and dilutes the credit when it really was earned.  The team can get into a habit of not taking the agreements seriously enough to do the last few millimeters of work that often turn out to be the hardest.  Some people say the ScrumMaster shouldn't even allow the team to demonstrate work which doesn't meet the definition of done.

A related topic Ken Schwaber has become adamant about is the danger of a team creating an artificially high velocity by getting into "technical debt."  Doing this for too long will lead to a "design dead" (unmaintainable) product.  Teams should allow time for *refactoring* to keep the code quality high.