MDL-65204 phpunit: tweak constraint_object_is_equal_with_exceptions
In PHPUnit 7.x and above, IsEqual->value became private and, as far
as our with exceptions class inherits from it, we cannot access to
that anymore.
So, in order to avoid that, we are overriding the constructor, capturing
the original value for own use and forgetting.
A more formal, alternative, solution would be to make our
exceptional class to inherit from Constraint and make the
class a pure dispatcher to different constraints, with IsEqual being
just one of them.
But we followed the easiest path here. Not ideal, but efective.
