tests:Update 2nd CB test to change rect value

Modify ConfirmNoVLErrorWhenVkCmdClearAttachmentsCalledInSecondaryCB
positive test to change the rect value after the call to
vkCmdClearAttachments() in the secondary command buffer but before the
secondary CB is executed in vkCmdExecuteCommands().
This exposes a previous bug in validation layers where the pointer to
the VkClearRect was copied, but not the underlying rect itself.