QTest::toString benchmark: do not pass a template type parameter

Let it to be deduced, otherwise it may force instantiations which
are ill-formed.

Change-Id: I3ce674128d96a48ad6883e265734ff330645dd75
Reviewed-by: Marc Mutz <marc.mutz@qt.io>

tests/benchmarks/testlib/tostring/tst_tostring.cpp

@@ -70,7 +70,7 @@ void tst_toString::numeric()
     QFETCH(T, datum);
 
     QBENCHMARK {
-        auto tst = QTest::toString<T>(datum);
+        auto tst = QTest::toString(datum);
         delete [] tst;
     }
 }