The instance move tests were moving the instance from node pair (A,_) to
(B, A), and left it there. This patch makes sure that the first step
moves the instance to (B,A) but the second one back to (A,B), so that
the instance is left on the same primary node.

The original secondary node is lost though, if I read the code
correctly.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>