debug-gui fails the test suite when run as root
The test suite checks for $DISPLAY to be set but if (like in the usual case) the X server doesn't allow root to connect and the suite runs as root, gtk_init() fails.
This often appears as heisenbug because if you run xhost si:localuser:root to debug something else while the test suite runs in the background, the failure cannot be reproduced anymore until after the next X server restart...
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