vim-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 vim/vim#5957)

08f4157c5c

Fix missing error message when sort() compare function fails.
Cherry-pick a line in test_utf8.vim from patch 8.2.0448.
Cherry-pick builtin_function() change from patch 8.2.0595.

test/functional/legacy/eval_spec.lua

@@ -639,7 +639,7 @@ describe('eval', function()
   end)
 
   it('function name includes a colon', function()
-    eq('Vim(function):E128: Function name must start with a capital or "s:": b:test()\\nendfunction',
+    eq('Vim(function):E884: Function name cannot contain a colon: b:test()\\nendfunction',
        exc_exec(dedent([[
         function! b:test()
         endfunction]])))