tests: Fix clang warning about missing constructor to initialize const member.

Author: emilio

tests/headers/template.hpp

@@ -7,7 +7,7 @@ template<typename T, typename U> class Foo {
 };
 
 template<typename T> class B {
-    T m_member;
+    T m_member { 0 };
 };
 
 void bar(Foo<int, int> foo);