test: force QT_QPA_PLATFORM=offscreen for qtdragon

xvfb + xcb + xcb_egl was not enough for Ubuntu 24.04 rip-and-test:
qtvcp still segfaults during widget construction even with opencv
and qtwebengine paths quiet, and the same code passes on Debian
package-arch. Offscreen renders entirely in memory and exercises a
different Qt plugin entirely, dodging the xcb-stack instability.

scripts/linuxcnc itself forces QT_QPA_PLATFORM=xcb unless
LINUXCNC_OPENGL_PLATFORM is set to a non-glx value, so pin both.
Only qtdragon needs this; axis (Tk), touchy and gmoccapy (GTK) are
unaffected. Trade-off: no Phase 3 screenshot from qtdragon under
this config; Phase 3 would need an opt-out for offscreen tests.

Author: grandixximo

tests/ui-smoke/qtdragon/test.sh

@@ -6,6 +6,15 @@
 # linuxcnc tears down before our driver can do anything. Mirror the
 # config dir to a writable tmp location and patch LOG_FILE to be
 # rooted at $HOME so the log lands in a directory we can write to.
+#
+# Force the Qt offscreen platform plugin. qtvcp under xvfb + xcb on
+# Ubuntu 24.04 segfaults during widget construction (no backtrace);
+# Debian containers in the same CI matrix do not. Offscreen renders
+# entirely in memory, no X server needed (xvfb-run still wraps the
+# call so the rest of scripts/linuxcnc's X-display assumptions hold).
+# scripts/linuxcnc itself forces QT_QPA_PLATFORM=xcb unless
+# LINUXCNC_OPENGL_PLATFORM is set to something other than glx, so we
+# pin both env vars.
 set -u
 
 LIB_DIR="$(cd "$(dirname "$0")/../_lib" && pwd)"
@@ -17,5 +26,8 @@ cp -r "$SRC_DIR/." "$WORK_DIR/"
 sed -i 's|^LOG_FILE = qtdragon\.log$|LOG_FILE = ~/qtdragon.log|' \
     "$WORK_DIR/qtdragon_metric.ini"
 
+export LINUXCNC_OPENGL_PLATFORM=offscreen
+export QT_QPA_PLATFORM=offscreen
+
 exec "$LIB_DIR/run-gui.sh" "$WORK_DIR/qtdragon_metric.ini" \
     --run-program "$LIB_DIR/smoke.ngc" --expect-delta-mm 1,1,0