Use QBENCHMARK instead of a hard requirement on the machine's performance

https://build.kde.org/job/plasma-framework%20master%20kf5-qt5/PLATFORM=Linux,Variation=All,compiler=gcc/52/console
failed due to the CI machine being a bit slow/busy.

autotests/packagestructuretest.cpp

@@ -76,15 +76,11 @@ void PackageStructureTest::copyPerformance()
     // seed the cache first
     ps.filePath("mainscript");
 
-    QTime t;
-    t.start();
-
-    for (int i = 0; i < 100000; ++i) {
+    QBENCHMARK {
         Plasma::Package foo(ps);
         const QString bar = foo.filePath("mainscript");
+        Q_UNUSED(bar);
     }
-
-    QVERIFY(t.elapsed() < 400);
 }
 
 void PackageStructureTest::mutateAfterCopy()