patch 8.2.0638: MS-Windows: messages test fails

Problem:    MS-Windows: messages test fails.
Solution:   Clear environment variables.
diff --git a/src/testdir/test_messages.vim b/src/testdir/test_messages.vim
index 84808ef..3d7245d 100644
--- a/src/testdir/test_messages.vim
+++ b/src/testdir/test_messages.vim
@@ -9,6 +9,9 @@
     set nomore
     " Avoid the "message maintainer" line.
     let $LANG = ''
+    let $LC_ALL = ''
+    let $LC_MESSAGES = ''
+    let $LC_COLLATE = ''
 
     let arr = map(range(10), '"hello" . v:val')
     for s in arr