[cpp-qt] use ctest, fix warnings

[MartinDelille]

• [cpp-qt-client] Use ctest
• [cpp-qt-client] Fix warning about deprecated count() method

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@ravinikam @stkrwork @etherealjoy @muttleyxd

[MartinDelille]

I didn't manage to configure properly openssl for windows and created an issue here: jurplel/install-qt-action#177

[MartinDelille]

Hi @wing328! I wondered if you would have any feedback on the PR! Best!

[wing328]

LGTM. Thanks for the PR.