Update tests for new preferred name as ShaderViewportIndexLayerEXT

This reacts to a recent update to SPIRV-Headers

test/enum_string_mapping_test.cpp

@@ -240,8 +240,11 @@ INSTANTIATE_TEST_CASE_P(AllCapabilities, CapabilityTest,
        "SampleMaskOverrideCoverageNV"},
       {SpvCapabilityGeometryShaderPassthroughNV,
        "GeometryShaderPassthroughNV"},
+      // The next two are different names for the same token.
       {SpvCapabilityShaderViewportIndexLayerNV,
-       "ShaderViewportIndexLayerNV"},
+       "ShaderViewportIndexLayerEXT"},
+      {SpvCapabilityShaderViewportIndexLayerEXT,
+       "ShaderViewportIndexLayerEXT"},
       {SpvCapabilityShaderViewportMaskNV,
        "ShaderViewportMaskNV"},
       {SpvCapabilityShaderStereoViewNV,