What caused me quite a bit of headache yesterday,was that, with a cleaned build dir, ninja tst_moc_check tst_qmake_check fails, because external tools (uic, qtpaths, qmake, ...) have not been built, yet. While the dependencies may be correct for the tst_moc and tst_qmake targets, they're definitely missing for the _check versions. At some point, I even had to ninja libQt6Widget.so manually, don't remember why, probably uic.

I guess the qmake build system didn't run into this problem because src/ was CONFIG+=ORDERED w.r.t. tests/.