We often do isEqual tests but where a field, may have a single exception.
Ordinarily we can handle this easily by breaking the test down, but when
dealing with mocked objects we can only pass an instance of a
PHPUnit_Framework_Constraint, or a single value (which is cast to an
instance of PHPUnit_Framework_Constraint_IsEqual).

To deal with instances of time-based exception, we need to be able to
compare simple objects and classes where one or two values may be
different.