Tests/Contracts: Update VTK repository URL

The vtk.org/vtk.git repository has long been superseded by the
repository on gitlab.kitware.com.  The former is no longer available.

Tests/Contracts/VTK/CMakeLists.txt

@@ -21,7 +21,7 @@ configure_file(
 
 # build & test VTK's release branch
 ExternalProject_Add(${PROJECT_NAME}
-  GIT_REPOSITORY "git://vtk.org/VTK.git"
+  GIT_REPOSITORY "https://gitlab.kitware.com/vtk/vtk.git"
   GIT_TAG "release"
   PREFIX ${base_dir}
   CONFIGURE_COMMAND ""