Stabilize tst_download.qml

Connect to WebEngineViewDownloadItem.stateChanged to collect state
changes. This makes sure that state changes from tests that ran before
do not interfere with the current test.

Change-Id: I360ac325711bf041f82375b3714a9cfe7a42398c
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@theqtcompany.com>