[App] Abort if no tests for reference solutions.

Abort if test definitions exist but no tests were actually
constructed from the configuration.

Applications/ApplicationsLib/TestDefinition.cpp

@@ -287,4 +287,9 @@ std::vector<std::string> const& TestDefinition::getOutputFiles() const
 {
     return _output_files;
 }
+
+std::size_t TestDefinition::numberOfTests() const
+{
+    return size(_command_lines);
+}
 }  // namespace ApplicationsLib

Applications/ApplicationsLib/TestDefinition.h

@@ -34,6 +34,7 @@ public:
 
     bool runTests() const;
     std::vector<std::string> const& getOutputFiles() const;
+    std::size_t numberOfTests() const;
 
 private:
     std::vector<std::string> _command_lines;

Applications/CLI/ogs.cpp

@@ -68,7 +68,8 @@ int main(int argc, char* argv[])
     TCLAP::ValueArg<std::string> reference_path_arg(
         "r", "reference",
         "Run output result comparison after successful simulation comparing to "
-        "all files in the given path.",
+        "all files in the given path. This requires test definitions to be "
+        "present in the project file.",
         false, "", "PATH");
     cmd.add(reference_path_arg);
 
@@ -204,6 +205,12 @@ int main(int argc, char* argv[])
                         project_config->getConfigSubtree("test_definition"),
                         reference_path_arg.getValue(),
                         outdir_arg.getValue());
+                if( test_definition->numberOfTests() == 0)
+                {
+                    OGS_FATAL(
+                        "No tests were constructed from the test definitions, "
+                        "but reference solutions path was given.");
+                }
 
                 INFO("Cleanup possible output files before running ogs.");
                 BaseLib::removeFiles(test_definition->getOutputFiles());