Add test for "qt" debug config property

[int19h]

See #308

Need to figure out how to properly do this in a non-UI test. QThread is in QtCore, so it should be doable.

[fabioz]

As a note, you can use the code in pydevd as a reference.

i.e.:

https://github.com/microsoft/debugpy/blob/master/src/debugpy/_vendored/pydevd/tests_python/resources/_debugger_case_qthread1.py
https://github.com/microsoft/debugpy/blob/master/src/debugpy/_vendored/pydevd/tests_python/resources/_debugger_case_qthread2.py
https://github.com/microsoft/debugpy/blob/master/src/debugpy/_vendored/pydevd/tests_python/resources/_debugger_case_qthread3.py
https://github.com/microsoft/debugpy/blob/master/src/debugpy/_vendored/pydevd/tests_python/resources/_debugger_case_qthread4.py

Although if I remember well, I think that even in this case I had to use (in travis):
export DISPLAY=:99.0
services: xvfb