patch 8.2.1356: Vim9: cannot get the percent register

Problem:    Vim9: cannot get the percent register.
Solution:   Check for readable registers instead of writable. (closes #6566)

src/testdir/test_vim9_expr.vim

@@ -1430,6 +1430,15 @@ enddef
 def Test_expr7_register()
   @a = 'register a'
   assert_equal('register a', @a)
+
+  let fname = expand('%')
+  assert_equal(fname, @%)
+
+  feedkeys(":echo 'some'\<CR>", "xt")
+  assert_equal("echo 'some'", @:)
+
+  normal axyz
+  assert_equal("xyz", @.)
 enddef
 
 def Test_expr7_namespace()