CppunitTest_sc_tiledrendering: std::string::find -> OString::startsWith

Change-Id: Ic7185cc0430e7007b1e4f27ba5c43d947405e198
Reviewed-on: https://gerrit.libreoffice.org/29292Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Jenkins <ci@libreoffice.org>

sc/qa/unit/tiledrendering/tiledrendering.cxx

@@ -440,8 +440,8 @@ public:
         break;
         case LOK_CALLBACK_INVALIDATE_TILES:
         {
-            std::string text(pPayload);
-            if (text.find("EMPTY") != std::string::npos)
+            OString text(pPayload);
+            if (text.startsWith("EMPTY"))
             {
                 m_bFullInvalidateTiles = true;
             }