testcon: Fix "unsafe" connect

In preparation for QT_ENABLE_STRICT_MODE the connect call was cleaned up
and the 4 parameter version is used instead of the 3 parameter version.
The actionFileExit's triggered signal will always be emitted by the
thread that qApp lives in, so AutoConnection will always result in a
direct activation.

Task-number: QTBUG-145279
Pick-to: 6.11
Change-Id: I22e568d461c3688c050762cbc71517158b3214be
Reviewed-by: Marc Mutz <marc.mutz@qt.io>
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>