Also de-duplicate the "monospace" string in qgenericunixthemes.cpp,
and add tst_QFontDatabase::systemFixedFont() to verify that
QFontDatabase::systemFont(QFontDatabase::FixedFont) really returns
a monospace font across platforms. Replace commented-out qDebug()s
with qt.text.font.match and qt.text.font.db logging categories to
troubleshoot when the test fails (among other uses). Add qt.qpa.fonts
logging category to unix themes to show default system and fixed fonts
(font engines on other platforms are already using this category).
Fixes: QTBUG-54623
Change-Id: I2aa62b8c783d9ddb591a5e06e8df85c4af5bcb0c
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>
