C++: Add a comment to a test file.

Author: MathiasVP

cpp/ql/test/library-tests/dataflow/external-models/azure.cpp

@@ -247,6 +247,8 @@ void test_RawResponse(Azure::Core::Http::RawResponse& resp) {
 }
 
 void test_GetHeader() {
+  // We instantiate the `std::pair` class since we need the `FieldContent` for
+  // `std::pair::second` to be present in the test DB.
   std::pair<std::string, std::string>();
   Azure::Core::Http::Request request(Azure::Core::Http::Get, Azure::Core::Url("http://example.com"));
   {