gui: add test runner summary

Summary:

Append a one-line summary to the output of running ./src/qt/test/test_bitcoin-qt indicating that all tests passed or showing the number of failing tests. It's currently a bit inconvenient to see this result by eyeballing all of the output.

This PR also adds an #include <QDebug> in test_main.cpp which was missed in D18200 when introducing a qWarning() << .... From the Qt doc:

If you include <QtDebug>, a more convenient syntax is also available
qWarning() << "..."`

This is a backport of core-gui#576

Depends on D18204

Test Plan:

ninja test_bitcoin-qt
src/qt/test/test_bitcoin-qt

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18205