patch 8.2.3522: cannot use \x and \u when setting 'listchars'

Problem:    Cannot use \x and \u when setting 'listchars'.
Solution:   Support hex and unicode in hex form. (closes #9006)
diff --git a/src/testdir/test_listchars.vim b/src/testdir/test_listchars.vim
index 4cbd365..cb947aa 100644
--- a/src/testdir/test_listchars.vim
+++ b/src/testdir/test_listchars.vim
@@ -288,6 +288,10 @@
   call cursor(1, 1)
   call assert_equal(expected, ScreenLines(1, virtcol('$')))
 
+  set listchars=eol:\\u21d4,space:\\u2423,multispace:≡\\u2262\\U00002263,nbsp:\\U00002260,tab:←↔\\u2192
+  redraw!
+  call assert_equal(expected, ScreenLines(1, virtcol('$')))
+
   set listchars+=lead:⇨,trail:⇦
   let expected = ['⇨⇨⇨⇨⇨⇨⇨⇨a←↔↔↔↔↔→b␣c≠d⇦⇦⇔']
   redraw!