Close memory leak in QTestLib support for string to numeric testing

QTest::toString allocates memory by calling qstrdup; that memory must
be freed by the caller.

Change-Id: I9724db0c617884cf08592968b1411f002ef23204
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>

tests/benchmarks/testlib/tostring/tst_tostring.cpp

@@ -95,7 +95,8 @@ void tst_toString::numeric()
     QFETCH(T, datum);
 
     QBENCHMARK {
-        QTest::toString<T>(datum);
+        auto tst = QTest::toString<T>(datum);
+        delete [] tst;
     }
 }