[Development] Windows QtTest changes free for the taking

[Thiago Macieira]

https://codereview.qt-project.org/c/qt/qtbase/+/413169/4
https://codereview.qt-project.org/c/qt/qtbase/+/412830/8

qtestcase.cpp has some support for printing backtraces on Windows using 
Windows API. Those two changes make some updates there.

Unfortunately, tst_selfest is undebuggable because it's not QtTest and I don't 
have a Windows machine. Therefore, I have no way of fixing the CI failure. So 
those two changes are free for the taking.

If no one adopts them in 2 weeks, I'll just abandon them in Gerrit.
-- 
Thiago Macieira - thiago.macieira (AT) intel.com
  Cloud Software Architect - Intel DCAI Cloud Engineering