patch 8.2.0610: some tests are still old style

Problem:    Some tests are still old style.
Solution:   Convert to new style tests. (Yegappan Lakshmanan, closes #5957)

src/testdir/test_utf8.vim

@@ -103,6 +103,9 @@ func Test_list2str_str2list_latin1()
   let sres = list2str(l, 1)
   call assert_equal([65, 66, 67], str2list("ABC"))
 
+  " Try converting a list to a string in latin-1 encoding
+  call assert_equal([1, 2, 3], str2list(list2str([1, 2, 3])))
+
   let &encoding = save_encoding
   call assert_equal(l, lres)
   call assert_equal(s, sres)