QtIFW

Fix cancel button functionality in Settings->Repositories->Test

When pressing cancel button when testing repositories, the dialog
pops up again and again. Cancel the TestRepository job if cancel
button is pressed.

Change-Id: I48da41a8b791bf04571acd8af4a421b11e0fccc7
Task-number: QTIFW-832
Reviewed-by: Niels Weber <niels.weber@qt.io>
Reviewed-by: Iikka Eklund <iikka.eklund@qt.io>