ENH: Mark test status helpers [[nodiscard]]

Test sources define file-scope helpers returning EXIT_SUCCESS or
EXIT_FAILURE, and a caller that drops the value turns every assertion
inside the helper into a no-op. Marking the helpers [[nodiscard]] makes
the compiler reject that.

Annotate the 290 declarations of such helpers and propagate the status
at the 30 call sites the attribute exposes, in eight tests spanning
image functions, smoothing, diffusion, XML, metrics and superpixel
segmentation.

The attribute goes on every declaration, not just the definition: it
only governs calls that follow the declaration carrying it, so a bare
forward declaration leaves earlier call sites unchecked.

Helpers whose int return is a value rather than a status are left
unannotated, as are the few using a bare 0/1 convention.

Author: hjmjohnson

Modules/Core/Common/test/SparseLUSolverTraitsGTest.cxx

@@ -47,7 +47,7 @@ VectorsEquals(const TVector & v1, const TVector & v2, const typename TVector::Sc
 
 namespace
 {
-int
+[[nodiscard]] int
 DoSparseLUSolverTraitsTest(int, char *[])
 {
   using CoordinateType = double;

Modules/Core/Common/test/VNLSparseLUSolverTraitsGTest.cxx

@@ -51,7 +51,7 @@ VectorsEquals(const TVector & v1, const TVector & v2, const typename TVector::el
 
 namespace
 {
-int
+[[nodiscard]] int
 DoVNLSparseLUSolverTraitsTest(int, char *[])
 {
   /**

Modules/Core/Common/test/itkBSplineInterpolationWeightFunctionGTest.cxx

@@ -43,7 +43,7 @@ static_assert((itk::BSplineInterpolationWeightFunction<float, 2, 1>::NumberOfWei
  */
 namespace
 {
-int
+[[nodiscard]] int
 DoBSplineInterpolationWeightFunctionTest(int, char *[])
 {
 

Modules/Core/Common/test/itkColorTableTest.cxx

@@ -23,7 +23,7 @@
 
 
 template <typename TComponent>
-int
+[[nodiscard]] int
 ColorTableTestExpectedConditionChecker(typename itk::ColorTable<TComponent>::Pointer colors,
                                        unsigned int                                  colorId,
                                        itk::RGBPixel<TComponent>                     rgbPixel,
@@ -62,7 +62,7 @@ ColorTableTestExpectedConditionChecker(typename itk::ColorTable<TComponent>::Poi
 }
 
 template <typename TComponent>
-int
+[[nodiscard]] int
 ColorTableTestSpecialConditionChecker(typename itk::ColorTable<TComponent>::Pointer colors, unsigned int numberOfColors)
 {
   using RGBPixelType = itk::RGBPixel<TComponent>;
@@ -132,7 +132,7 @@ ColorTableTestSpecialConditionChecker(typename itk::ColorTable<TComponent>::Poin
 }
 
 template <typename TComponent>
-int
+[[nodiscard]] int
 ColorTableTestHelper(const char * name, unsigned int numberOfColors)
 {
   int testStatus = EXIT_SUCCESS;

Modules/Core/Common/test/itkConicShellInteriorExteriorSpatialFunctionGTest.cxx

@@ -24,7 +24,7 @@
 
 namespace
 {
-int
+[[nodiscard]] int
 DoConicShellInteriorExteriorSpatialFunctionTest(int, char *[])
 {
 

Modules/Core/Common/test/itkConstNeighborhoodIteratorWithOnlyIndexTest.cxx

@@ -38,7 +38,7 @@ itkConstNeighborhoodIteratorWithOnlyIndexTestGetTestImage(int d1, int d2, int d3
 }
 
 template <typename TImage>
-int
+[[nodiscard]] int
 itkConstNeighborhoodIteratorWithOnlyIndexTestRun()
 {
   const typename TImage::Pointer img = itkConstNeighborhoodIteratorWithOnlyIndexTestGetTestImage<TImage>(10, 10, 5, 3);

Modules/Core/Common/test/itkEllipsoidInteriorExteriorSpatialFunctionGTest.cxx

@@ -21,7 +21,7 @@
 
 namespace
 {
-int
+[[nodiscard]] int
 DoEllipsoidInteriorExteriorSpatialFunctionTest(int, char *[])
 {
   std::cout << "itkEllipsoidInteriorExteriorSpatialFunction test start" << std::endl;

Modules/Core/Common/test/itkFileOutputWindowGTest.cxx

@@ -24,7 +24,7 @@
 
 namespace
 {
-int
+[[nodiscard]] int
 DoFileOutputWindowTest(int, char *[])
 {
 

Modules/Core/Common/test/itkFiniteCylinderSpatialFunctionGTest.cxx

@@ -22,7 +22,7 @@
 
 namespace
 {
-int
+[[nodiscard]] int
 DoFiniteCylinderSpatialFunctionTest(int, char *[])
 {
   std::cout << "itkFiniteCylinderSpatialFunction test start" << std::endl;

Modules/Core/Common/test/itkFrustumSpatialFunctionGTest.cxx

@@ -24,7 +24,7 @@
 
 namespace
 {
-int
+[[nodiscard]] int
 DoFrustumSpatialFunctionTest(int, char *[])
 {