test: add namespace for test helpers

tests/helpers/scoped_file.hpp

@@ -4,6 +4,8 @@
 #include <stdexcept>
 #include <string>
 
+namespace helpers {
+
 class ScopedFile {
 public:
   explicit ScopedFile(std::string fileName) : fileName_(std::move(fileName)) {}
@@ -25,3 +27,5 @@ public:
 private:
   std::string fileName_;
 };
+
+} // namespace helpers