Answer:
Acceptance testing and the benchmarks for the acceptance testing should be set out in the software development agreement. That way if there is any disagreement between the parties in relation to the acceptance testing you can make it clear to the developer that they are not meeting the obligations under the agreement.
If you disagree, you should bring this to the attention of the developer and not proceed with the next stage of development. You should then investigate dispute resolution if you cannot resolve the situation, or terminate the agreement if you can no longer work together.