avoid push_back in loops

Detected by clang-tidy.

test/job/queue.cpp

@@ -34,14 +34,12 @@ main (void)
         {
             util::job::queue q {std::thread::hardware_concurrency (), INNER};
             std::vector<util::job::queue::cookie> cookies;
-            for (int j = 0; j < INNER; ++j) {
-                cookies.push_back (
-                    q.submit (
-                        sleep_inc,
-                        std::ref (count)
-                    )
+            std::generate_n (std::back_inserter (cookies), INNER, [&] () {
+                return q.submit (
+                    sleep_inc,
+                    std::ref (count)
                 );
-            }
+            });
         }
 
         success = count == INNER && success;

test/thread/flag.cpp

@@ -60,8 +60,13 @@ main ()
         };
 
         std::vector<std::thread> workers;
-        for (int i = 0; i < parallelism; ++i)
-            workers.emplace_back (func, i);
+        std::generate_n (
+            std::back_inserter (workers),
+            parallelism,
+            [&, i = 0] () mutable
+        {
+               return std::thread { func, i++ };
+        });
 
         for (auto &t: workers)
             t.join ();