AutoTest: Support gathering failed tests

Mark test tree items as failed for the last run
to be able to re-run them in an easier way.

Change-Id: I7ea3dcd16e5a02797d41f13e02b2fa95b857cf5e
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/autotest/testrunner.cpp

@@ -335,6 +335,7 @@ void TestRunner::prepareToRunTests(TestRunMode mode)
 
     // clear old log and output pane
     TestResultsPane::instance()->clearContents();
+    TestTreeModel::instance()->clearFailedMarks();
 
     if (m_selectedTests.empty()) {
         reportResult(ResultType::MessageWarn, tr("No tests selected. Canceling test run."));