patch 8.2.2724: Vim9: concatenating to list in dict not tested

Problem:    Vim9: concatenating to list in dict not tested.
Solution:   Add a test. (issue #8068)

src/testdir/test_vim9_assign.vim

@@ -1152,6 +1152,10 @@ def Test_assign_dict_with_op()
     dd.a.b = [0]
     dd.a.b += [1]
     assert_equal({a: {b: [0, 1]}}, dd)
+
+    var dab = {a: ['b']}
+    dab.a[0] ..= 'c'
+    assert_equal({a: ['bc']}, dab)
   END
   CheckDefAndScriptSuccess(lines)
 enddef