Some instance test functions took two node arguments, some took one, and
some took two but the second argument could be None. This patch makes such
functions uniform by using a list of nodes as an argument.  This simplifies
the following refactoring.

No test logic has been changed.

Signed-off-by: Bernardo Dal Seno <bdalseno@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>