[cleanup] Mark loop variable as unused

Here the purpose of the loop is to do some operation several
times, but the operation does not depend on the loop variable
itself.

dune/fufem/test/loggertest.cc

@@ -50,7 +50,7 @@ auto checkLogger(std::string format, std::string prefixRegex)
     auto threads = std::vector<std::thread>();
     for(auto i: Dune::range(3))
       threads.emplace_back([&,i]() {
-        for(auto i: Dune::range(3))
+        for([[maybe_unused]] auto i: Dune::range(3))
         {
           using namespace std::chrono_literals;
           std::this_thread::sleep_for(10ms);
@@ -60,7 +60,7 @@ auto checkLogger(std::string format, std::string prefixRegex)
     for(auto& thread: threads)
       thread.join();
     auto regex = std::string();
-    for(auto i: Dune::range(9))
+    for([[maybe_unused]] auto i: Dune::range(9))
       regex = regex+prefixRegex+"foo\n";
     testSuite.check(std::regex_match(sstream.str(), std::regex(regex)))
       << "Non-matching string generated by synchronized logger with format '" << format <<"'";