fix for mock_exchange winprice amount

We should use ```MicroUSD_CPM``` because ```FixedPriceBiddingAgent``` use it

```
            // Create a 0.0001$ CPM bid with our available creative.
            // Note that by default, the bid price is set to 0 which indicates
            // that we don't wish to bid on the given spot.
            bid.bid(availableCreative, MicroUSD_CPM(10000));
```

otherwise ```winPrice``` can be greater than ```bidPrice```