Skip flaky subtests of tests/auto/qmltest.

- Skip test_key_click() in events/tst_events.qml.
- Skip test_pixel pixel/tst_pixel.qml.

Task-number: QTBUG-27671
Change-Id: Id78c34244d346910e28a2368f9fe3a41f54b8fb9
Reviewed-by: Janne Anttila <janne.anttila@digia.com>
Reviewed-by: Stephen Kelly <stephen.kelly@kdab.com>