TaskTree: Introduce For element

Improve code readability by requiring iteratable Group
to be named For.

The For's c'tor requires an iterator element as a first arg.

The For's c'tor allows for passing exactly one iterator element.

It's not possible to place iterators inside Group element anymore.

Change-Id: I9dfe2c0da058abac161f66c4e336da2417c383f1
Reviewed-by: hjk <hjk@qt.io>

src/plugins/autotest/testrunner.cpp

@@ -459,15 +459,15 @@ void TestRunner::runTestsHelper()
         }
     };
 
-    const Group root {
-        finishAllAndSuccess,
+    const For recipe {
         iterator,
+        finishAllAndSuccess,
         Group {
             storage,
             ProcessTask(onSetup, onDone)
         }
     };
-    m_taskTreeRunner.start(root);
+    m_taskTreeRunner.start(recipe);
 }
 
 static void processOutput(TestOutputReader *outputreader, const QString &msg, OutputFormat format)