qt6, test: Handle deprecated code

Summary:

qt6, test: Use qWarning() instead of QWARN() macro

The QWARN() macro internally uses QTest::qWarn(), which has been
deprecated since Qt 6.3. Replacing it with qWarning() ensures
compatibility across all Qt versions.

qt6, test: Handle deprecated QVERIFY_EXCEPTION_THROWN

This change ensures compatibility across all supported Qt versions.

Co-Authored-By: João Barbosa <joao.paulo.barbosa@gmail.com>

This is a backport of core-gui#839
Depends on D18199

Test Plan:
ninja check
With Qt6.4, verify the lack of deprecation warnings when compiling

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18200