Add -Wmissing-declarations to the SelfTest project

This required some clean-up in our test files

projects/SelfTest/UsageTests/Decomposition.tests.cpp

@@ -9,6 +9,8 @@
 #include <iostream>
 #include <cstdio>
 
+namespace {
+
 struct truthy {
     truthy(bool b):m_value(b){}
     operator bool() const {
@@ -22,6 +24,8 @@ std::ostream& operator<<(std::ostream& o, truthy) {
     return o;
 }
 
+} // end anonymous namespace
+
 #include "catch.hpp"
 
 TEST_CASE( "Reconstruction should be based on stringification: #914" , "[Decomposition][failing][.]") {