tst_qquickvisualdatamodel randomly fails

First - I don't know if I choose correct component.

I can't submit one change for few days:
https://codereview.qt-project.org/#change,60170

Change fails on tst_qquickvisualdatamodel test.

After asking on qt-qa irc tosoraja gave me some bigger perspective:

[12:38] <tosaraja> tolszak_work: a moment...i'll check something
[12:42] <tosaraja> tolszak_work: during the last 2 weeks that test case has failed 1 time before your build. Your build numbers are 488, 489 and 491. 486 is some other build and that had already failed that case. i'll go see which build that is
[12:47] <tosaraja> tolszak_work: this task was the first one that initially failed that test case: https://codereview.qt-project.org/#change,59524
[12:47] <tosaraja> tolszak_work: after a restage, that commit then passed the build and was merged. The next builds after that began failing again though
[12:50] <tosaraja> tolszak_work: the whole tst_qquickvisualdatamodel has failed 56 times during the last 2 weeks and 591 times since last february It isn't the most stable test set we have