QBenchmarkResult: fix uninitialized value

This manifested itself in a UBSan report about loading a non-enumerated
value from an enumeration variable:

   src/testlib/qbenchmark_p.h:95:7: runtime error: load of value 11091, which is not a valid value for type 'QBenchmarkMetric'

(or similar).

The chosen value is simply QTest::QBenchmarkMetric(0).

Change-Id: I8492a871a71d89fa6f7902d38f9eecee4b060646
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>

src/testlib/qbenchmark_p.h

@@ -105,6 +105,7 @@ public:
     QBenchmarkResult()
     : value(-1)
     , iterations(-1)
+    , metric(QTest::FramesPerSecond)
     , setByMacro(true)
     , valid(false)
     { }