Generally, in cases like this, you can create a wrapper interface which you can use to stub the GetUser() method.
public interface IMembersipService {
MembershipUser GetUser();
}
public class MembersipService : IMembersipService
{
public MembershipUser GetUser() {
return Membership.GetUser();
}
}
IMembersipService.GetUser() can be stub/mock based on your need.
Additional note: there are other routines in the Membership class, which you may required to access. You can use the same interface to access wrap those methods. Having a wrapped interface like this around static members provide the testability.