QA: Use lists of nodes as argument to instance tests

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>