Fix minimum benchmark time with multiple threads

Closes #2117

Author: hahnjo

src/benchmark_runner.cc

@@ -361,6 +361,9 @@ BenchmarkRunner::IterationResults BenchmarkRunner::DoNIterations() {
   } else if (b.use_real_time()) {
     i.seconds = i.results.real_time_used;
   }
+  // The time is the sum over all threads, we want each of them to run the
+  // specificed minimum time.
+  i.seconds /= b.threads();
 
   return i;
 }