Fix tests to work with Coverage 4.0
Fix tests to work with Coverage 4.0

Per the change:
https://github.com/nedbat/coveragepy/commit/a2a2c1f3f41b0fc4afef0fe221499aedca343de8
Coverage can't append to data files in parallel mode.

This change removes the "--append" flag when running Coverage.

RafaelPalomar/girder,jbeezley/girder,msmolens/girder,Xarthisius/girder,Xarthisius/girder,msmolens/girder,RafaelPalomar/girder,Xarthisius/girder,manthey/girder,adsorensen/girder,sutartmelson/girder,kotfic/girder,adsorensen/girder,msmolens/girder,manthey/girder,girder/girder,data-exp-lab/girder,kotfic/girder,essamjoubori/girder,manthey/girder,RafaelPalomar/girder,salamb/girder,essamjoubori/girder,adsorensen/girder,sutartmelson/girder,RafaelPalomar/girder,salamb/girder,sutartmelson/girder,Kitware/girder,essamjoubori/girder,Kitware/girder,sutartmelson/girder,manthey/girder,jbeezley/girder,girder/girder,msmolens/girder,Xarthisius/girder,salamb/girder,salamb/girder,essamjoubori/girder,Kitware/girder,data-exp-lab/girder,adsorensen/girder,Xarthisius/girder,salamb/girder,girder/girder,data-exp-lab/girder,kotfic/girder,essamjoubori/girder,girder/girder,data-exp-lab/girder,kotfic/girder,Kitware/girder,jbeezley/girder,data-exp-lab/girder,msmolens/girder,jbeezley/girder,adsorensen/girder,RafaelPalomar/girder,sutartmelson/girder,kotfic/girder