AutoTest: Restore former check state on reparse

When reparsing while modifying a file it can happen that
the parse failed for some reason or did not provide full
information due to syntax errors the code model cannot cope
with.
This in turn can purge items from the test tree. Re-adding
the items in a later reparse had just added the item and
did not take care of (former) check states.
Add simple caching mechanism to keep track of check states
and use them if available.

Task-number: QTCREATORBUG-24099
Change-Id: I3ca04f5fd58810df71582972e6fe96a00cfc48f1
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/autotest/testcodeparser.cpp

@@ -334,6 +334,7 @@ void TestCodeParser::scanForTests(const QStringList &fileList, const QList<ITest
     }
 
     parsingHasFailed = false;
+    TestTreeModel::instance()->updateCheckStateCache();
     if (isFullParse) {
         // remove qml files as they will be found automatically by the referencing cpp file
         list = Utils::filtered(list, [] (const QString &fn) {