Add -Wmissing-declarations to the SelfTest project

This required some clean-up in our test files

projects/SelfTest/UsageTests/ToStringGeneral.tests.cpp

@@ -116,6 +116,8 @@ TEST_CASE("Static arrays are convertible to string", "[toString]") {
     }
 }
 
+namespace {
+
 struct WhatException : std::exception {
     char const* what() const noexcept override {
         return "This exception has overriden what() method";
@@ -136,6 +138,8 @@ struct StringMakerException : std::exception {
     ~StringMakerException() override;
 };
 
+} // end anonymous namespace
+
 namespace Catch {
 template <>
 struct StringMaker<StringMakerException> {