ENH: Change default TEST_LABELS for module
Currently, the TEST_LABELS for a vtk module defaults to the empty string. Currently, only one VTK module sets TEST_LABELS in its module.cmake file (vtkWebCore). This patch sets the default to the name of the module. This facilitates testing of individual modules.
For example, when using the testing script vtk_common.cmake, one can set
set(MODULE "vtkCommonCore")
set(CTEST_TEST_ARGS INCLUDE_LABEL ${MODULE})
and the testing script will only test those tests which have the label vtkCommonCore. This is especially useful for remote modules. Rather than running all of vtk's tests, a test script would just run the remote module's tests.