patch 8.2.4200: some tests do not clean up properly

Problem:    Some tests do not clean up properly.
Solution:   Delete created files. (Yegappan Lakshmanan, closes #9611)
diff --git a/src/testdir/test_messages.vim b/src/testdir/test_messages.vim
index 1ba92ff..d3c80bf 100644
--- a/src/testdir/test_messages.vim
+++ b/src/testdir/test_messages.vim
@@ -373,6 +373,7 @@
   " clean up
   call StopVimInTerminal(buf)
   call delete('Xtest_fileinfo_after_echo')
+  call delete('b.txt')
 endfunc
 
 " vim: shiftwidth=2 sts=2 expandtab