[tests] Ignore *.dfont files

It seems that Travis has fc-list installed and we end up with *.dfont
files in the system list of fonts, but we don’t support these.

tests/test_good_fonts.sh

@@ -50,6 +50,7 @@ if [ $# -eq 0 ] ; then
   FAILS=0
   IFS=$'\n'
   for f in $FONTS; do
+    if [[ $f == *.dfont ]]; then continue; fi # Ignore .dfont’s
     $0 "$f"
     FAILS=$((FAILS+$?))
   done