Stop using gtk-missing-image
I finally got the build tests for glade a bit closer to running in Debian which uncovered a bug.
Please stop using the gtk-missing-image icon. GNOME considers it deprecated ever since they switched from gnome-icon-theme to adwaita-icon-theme and dropped a bunch of former "stock" icons.
Workaround
Install gnome-icon-theme
Build log excerpt
========================================
glade 3.22.1: tests/test-suite.log
========================================
# TOTAL: 3
# PASS: 0
# SKIP: 0
# XFAIL: 0
# FAIL: 3
# XPASS: 0
# ERROR: 0
.. contents:: :depth: 2
FAIL: create-widgets
====================
GladeUI-Message: 12:46:46.008: 1 missing displayable value for GtkWidget::events
GladeUI-Message: 12:46:46.009: 1 missing displayable value for GtkWidget::AtkObject::accessible-role
(/<<PKGBUILDDIR>>/tests/.libs/create-widgets:31796): GladeUI-WARNING **: 12:46:46.032:
Unable to render icon gtk-missing-image at size 16: Icon 'gtk-missing-image' not present in theme Adwaita
FAIL create-widgets (exit status: 133)
FAIL: add-child
===============
GladeUI-Message: 12:46:46.061: 1 missing displayable value for GtkWidget::events
GladeUI-Message: 12:46:46.062: 1 missing displayable value for GtkWidget::AtkObject::accessible-role
(/<<PKGBUILDDIR>>/tests/.libs/add-child:31797): GladeUI-WARNING **: 12:46:46.085:
Unable to render icon gtk-missing-image at size 16: Icon 'gtk-missing-image' not present in theme Adwaita
FAIL add-child (exit status: 133)
FAIL: toplevel-order
====================
GladeUI-Message: 12:46:46.042: 1 missing displayable value for GtkWidget::events
GladeUI-Message: 12:46:46.043: 1 missing displayable value for GtkWidget::AtkObject::accessible-role
(/<<PKGBUILDDIR>>/tests/.libs/toplevel-order:31798): GladeUI-WARNING **: 12:46:46.070:
Unable to render icon gtk-missing-image at size 16: Icon 'gtk-missing-image' not present in theme Adwaita
FAIL toplevel-order (exit status: 133)