QBenchlib: force the warmup to run a single iteration

The purpose of warming up is to get all code paths executed, so any lazy
function resolving is processed, statics are allocated, etc. There's no
reason to run it more than once -- if you're trying to train the Branch
Predictor Unit, you'd want to do it another way anyway. This is useful
when benchmarking with -iterations N, because QBenchlib currently runs
2*N iterations because of the warm up. That just wastes time.

Change-Id: I3c79b7e08fa346988dfefffd172030c889b31a1f
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>

src/testlib/qtestcase.cpp

@@ -1117,6 +1117,8 @@ void TestMethods::invokeTestOnData(int index) const
     bool minimumTotalReached = false;
     do {
         QBenchmarkTestMethodData::current->beginDataRun();
+        if (i < 0)
+            QBenchmarkTestMethodData::current->iterationCount = 1;
 
         /* Benchmarking: for each accumulation iteration*/
         bool invokeOk;