Fix MapFlick::test_flick_diagonal autotest

init() is currently setting zoomLevel to 0.
This prevents the map from flicking diagonally as there's no
room for that when the map is fully visible vertically.
This test is currently passing in CI for unclear reasons,
but it's failing locally.

Change-Id: I0bb7c703a6cd2d4b5f440ce629c3933cf41268f6
Reviewed-by: Alex Blasche <alexander.blasche@qt.io>