Unit testing docs (#3698)

* Updating library-tests and associated documentation to be more informative

* Fixing heading markup for updated unit testing docs

unit_tests/library/args.hpp

@@ -3,6 +3,7 @@
 
 #include <string>
 #include <vector>
+#include <iostream>
 
 inline std::vector<std::string> get_args()
 {
@@ -10,6 +11,11 @@ inline std::vector<std::string> get_args()
     const auto argc = boost::unit_test::framework::master_test_suite().argc - 1;
     const auto argv = boost::unit_test::framework::master_test_suite().argv + 1;
 
+    if (argc == 0)
+    {
+        std::cout << "You must provide a path to the test data, please see the unit testing docs" << std::endl;
+    }
+
     return {argv, argv + argc};
 }