QtCore's libraries (glib, icu) can load while QtWidgets/QtGui still
miss libGL - matplotlib's qt backend imports all three, so the old
probe could promise QtAgg and lose even the error figure at
switch_backend, one library layer deeper than the NixOS fix. The
regression test's mock now mirrors the real failure (bare package and
QtCore import fine, GUI modules raise): both the bare-import and the
QtCore-probe variants now fail it, verified by mutation. Also drop two
redundant f-prefixes on placeholder-less strings in report.py.
grabowski
de7a4f8641