Sprinkle some constexpr around to make Jason happy

Most of these will not matter in practice due to C++14 imposing
significant limitations on what else we can make constexpr, and we cannot
have references outliving the constexpr context either way.

src/catch2/internal/catch_test_registry.cpp

@@ -54,7 +54,7 @@ namespace Catch {
             TestType m_testAsFunction;
 
         public:
-            TestInvokerAsFunction( TestType testAsFunction ) noexcept:
+            constexpr TestInvokerAsFunction( TestType testAsFunction ) noexcept:
                 m_testAsFunction( testAsFunction ) {}
 
             void invoke() const override { m_testAsFunction(); }