Applied iwyu --comment to the code base (#588)

tests/owner_tests.cpp

@@ -14,12 +14,9 @@
 //
 ///////////////////////////////////////////////////////////////////////////////
 
-#include <catch/catch.hpp>
+#include <catch/catch.hpp> // for AssertionHandler, StringRef, CHECK, TEST_...
 
-#include <gsl/pointers>
-
-#include <functional>
-#include <memory>
+#include <gsl/pointers> // for owner
 
 using namespace gsl;