api: Convert client unit tests to use testify
api: Convert client unit tests to use testify

It's easier to read.

ejj/quilt-dev,kklin/kelda,kklin/kelda,kayousterhout/quilt,LuiseV/quilt,kelda/kelda,ejj/quilt-dev,kelda/kelda,kelda/kelda,NetSys/quilt,LuiseV/quilt,NetSys/quilt,kklin/quilt,ejj/quilt-dev,ejj/quilt-dev,kklin/kelda,LuiseV/quilt,quilt/quilt,kayousterhout/quilt,quilt/quilt,LuiseV/quilt,NetSys/quilt,kayousterhout/quilt,kklin/quilt,kelda/kelda,LuiseV/quilt,quilt/quilt,NetSys/quilt,kklin/kelda,quilt/quilt,kayousterhout/quilt,NetSys/quilt,kklin/quilt,kklin/quilt