ClangTools: Further increase the test timeout

The clazy tool in particular can be exceedingly slow on Windows.
Increase the default timeout and also add a hook to set the timeout at
runtime via the environment.

Change-Id: I94c46ce03f8348fb239870df4d5627bf8d545307
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/plugins/clangtools/clangtoolsunittests.h

@@ -56,8 +56,11 @@ private:
                     const CppTools::ClangDiagnosticConfig &diagnosticConfig);
 
 private:
+    static int getTimeout();
+
     CppTools::Tests::TemporaryCopiedDir *m_tmpDir = nullptr;
     ProjectExplorer::Kit *m_kit = nullptr;
+    int m_timeout = getTimeout();
 };
 
 } // namespace Internal