patch 9.1.0757: tests: messages files contains ANSI escape sequences

Problem:  tests: messages files contains ANSI escape sequences
Solution: filter those out in runtests.vim

Also, since we are setting $LC_ALL, we don't need to set $LANG and
$LANGUAGE since those are overridden by $LC_ALL anyhow.

closes: #15788

Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/testdir/runtest.vim b/src/testdir/runtest.vim
index 133993d..7ac4845 100644
--- a/src/testdir/runtest.vim
+++ b/src/testdir/runtest.vim
@@ -522,11 +522,11 @@
   " Add SKIPPED messages
   call extend(s:messages, s:skipped)
 
-  " Append messages to the file "messages"
+  " Append messages to the file "messages", but remove ANSI Escape sequences
   split messages
   call append(line('$'), '')
   call append(line('$'), 'From ' . g:testname . ':')
-  call append(line('$'), s:messages)
+  call append(line('$'), s:messages->map({_, val -> substitute(val, '\%x1b\[\d\?m', '', 'g')}))
   write
 
   qall!