test: SonarCloud issues of the unit tests

- test::definition_error for unusable test vehicle definitions instead of
  std::runtime_error (S112)
- the rows of a table section written by a function of their own, less
  nesting in definition::text() (S134)
- std::less<> for the section table (S6045)
- comments in the empty methods of the reseeding listener and stub (S1186)
- NOSONAR where the engine's headers fix the stubs: the extern globals
  and the member function signatures (S5421, S5817, S1238)
- NOSONAR on LoadFIZ_Parse(): the cognitive complexity is the former body
  of LoadFIZ(), moved as it was (S3776)
- a comment Sonar took for code reworded (S125)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/unit/engine_stubs.cpp

@@ -44,29 +44,32 @@ void ErrorLog(const std::string &str, logtype const Type) {
 }
 
 // scenery structure, which the parser reports include directives to --------------------------------
+// the declarations come from the engine's headers, the stubs can't change them (hence the NOSONARs)
 
 namespace scene {
 
-node_groups Groups;
-node_layers Layers;
+node_groups Groups; // NOSONAR the engine's extern global, scene/scenenodegroups.h
+node_layers Layers; // NOSONAR the engine's extern global, scene/scenelayers.h
 
-group_handle node_groups::create(bool const) {
+group_handle node_groups::create(bool const) { // NOSONAR signature from scene/scenenodegroups.h
 	return null_handle;
 }
 
-group_handle node_groups::close() {
+group_handle node_groups::close() { // NOSONAR signature from scene/scenenodegroups.h
 	return null_handle;
 }
 
-layer_handle node_layers::open(std::string const &, include_site) {
+layer_handle node_layers::open(std::string const &, include_site) { // NOSONAR signature from scene/scenelayers.h
 	return null_handle;
 }
 
-layer_handle node_layers::close() {
+layer_handle node_layers::close() { // NOSONAR signature from scene/scenelayers.h
 	return null_handle;
 }
 
-void node_layers::instance_begin(std::string const &, source_span const &) {}
+void node_layers::instance_begin(std::string const &, source_span const &) { // NOSONAR signature from scene/scenelayers.h
+	// no layers in the tests, so nothing to keep track of
+}
 
 // basic_node has no key function for the Itanium C++ ABI (tooltip() is declared plain but defined inline), so
 // GCC and Clang emit its vtable wherever scenenode.h is used, here too, and the vtable wants radius_().