BUG: ImageSources/test/CMakeLists.txt incorrect library var reference

The CreateTestDrivers command referred to ITKSource-Test_LIBRARIES,
when the correct variable name is ITKImageSources-Test_LIBRARIES.
This was never noticed because ImageSources only depends on
header-only modules, so both the correctly-named and
incorrectly-named CMake variable would be empty.

Change-Id: I86ab2998d173d8b196575f7a5f2277444f184b45
Merge topic 'ImageSourcesTestLibsFix'

7e9bf781 BUG: ImageSources/test/CMakeLists.txt incorrect library var reference