Disable particle system tests for CI

Disable particle system tests for CI

Particle System tests appear to be unstable when run in CI. They should
not have their results counted until this issue is resolved.

Task-number: QTBUG-21867

Change-Id: Ib6a7cd6a48f96bdb1558a1061e0952f583bbcd91
Reviewed-on: http://codereview.qt-project.org/6106
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Rohan McGovern <rohan.mcgovern@nokia.com>