vtkTestingMacros: Add vtk_add_test_cxx function

Create a function with a signature similar to vtk_add_test_(python|tcl)
that can be used to add C++-implemented tests.  Support more than one
test name per call each optionally followed by a comma-separated list of
per-test options.  Store test source files in a VTK_TEST_CXX_SOURCES
directory property so callers can get the list of source files to
compile into a test driver.

Add macro vtk_test_cxx_executable to get the VTK_TEST_CXX_SOURCES value
and invoke create_test_sourcelist and vtk_module_test_executable.  Use
optional arguments to enable the rendering factory or add extra sources
to the executable.

Change-Id: I73dee627fa87a399c48b409af046b116fc75a0fd