Qt: Keep dialogs from blocking tests.

If WIRESHARK_QUIT_AFTER_CAPTURE is set, call exit(0). This keeps us from
emitting the aboutToQuit signal and doing things that might pop up
dialogs.

Change-Id: I6e986b6e7ad824d8b5258e34a12c32869f5b2d34
Reviewed-on: https://code.wireshark.org/review/4421
Reviewed-by: Gerald Combs <gerald@wireshark.org>