AutoTest: Avoid crash on concurrent access

Partially reverts 91bdbb61ec as
the symbol lookup will update the snapshot's cache. As there
are concurrent threads accessing the snapshot we may end up in
a crash.

Change-Id: I2aaf5e403b10cba2b26915cbe7b4712c43c4957b
Reviewed-by: Marcus Tillmanns <marcus.tillmanns@qt.io>

src/plugins/autotest/qtest/qttestvisitors.h

@@ -34,7 +34,7 @@ public:
 private:
     CppEditor::SymbolFinder m_symbolFinder;
     QString m_className;
-    const CPlusPlus::Snapshot &m_snapshot;
+    CPlusPlus::Snapshot m_snapshot;
     QMap<QString, QtTestCodeLocationAndType> m_privSlots;
     bool m_valid = false;
     bool m_inherited = false;