AutoTest: Further optimize TestCodeParser::scanForTests()

When loading a Qt project, after the Scanning For Tests
finished, the scanForTests() blocks the main thread for
about 3.5 seconds on the calls to parser->init().

Refactor the code so that it operates on QSet<FilePath>
instead of QList<FilePaths>.

This patch constraints the freeze to about 40 ms.

Change-Id: I219b3e2abf2b7e5166eec08d83f4cdcb8e4a8098
Reviewed-by: Jarek Kobus <jaroslaw.kobus@qt.io>
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/plugins/autotest/qtest/qttestparser.h

@@ -34,7 +34,7 @@ class QtTestParser : public CppParser
 public:
     explicit QtTestParser(ITestFramework *framework) : CppParser(framework) {}
 
-    void init(const Utils::FilePaths &filesToParse, bool fullParse) override;
+    void init(const QSet<Utils::FilePath> &filesToParse, bool fullParse) override;
     void release() override;
     bool processDocument(QPromise<TestParseResultPtr> &promise,
                          const Utils::FilePath &fileName) override;