tests: Explicitly test 'default' style inside an array assignment.

highlighters/main/test-data/assign.zsh

@@ -33,5 +33,6 @@ expected_region_highlight=(
   "1 3 assign" # A=1
   "5 7 assign" # b=(
   "8 12 double-quoted-argument" # "foo"
+  "14 16 default" # bar
   "17 17 assign" # )
 )