Fix test configuration error
Fix test configuration error

This commit fixes an error when configuring Girder tests due to EXTERNAL_DATA
referencing nonexistent files. For example:

    vagrant@isic-archive:~/girder-build$ cmake --version
    cmake version 2.8.12.2

    vagrant@isic-archive:~/girder-build$ cmake ../girder
    ...
    -- Including plugin.cmake from "large_image"
    CMake Error at /usr/share/cmake-2.8/Modules/ExternalData.cmake:434 (message):
      Data file referenced by argument

	DATA{/home/vagrant/girder-build/data_src/sample_image.ptif}

      corresponds to source tree path

	sample_image.ptif

      that does not exist as a file (with or without an extension)!
    Call Stack (most recent call first):
      /usr/share/cmake-2.8/Modules/ExternalData.cmake:255 (_ExternalData_arg)
      tests/TestData.cmake:105 (ExternalData_Expand_Arguments)
      tests/PythonTests.cmake:153 (girder_ExternalData_expand_arguments)
      plugins/large_image/plugin.cmake:47 (add_python_test)
      tests/CMakeLists.txt:90 (include)
      tests/CMakeLists.txt:105 (_add_plugin)

In the Travis environment these errors show up as warnings.

DigitalSlideArchive/large_image,girder/large_image,girder/large_image,DigitalSlideArchive/large_image,girder/large_image,DigitalSlideArchive/large_image