patch 8.1.1546: in some tests 'tags' is set but not restored

Problem:    In some tests 'tags' is set but not restored. (Daniel Hahler)
Solution:   Restore 'tags'. (closes #4535)
diff --git a/src/testdir/test_cmdline.vim b/src/testdir/test_cmdline.vim
index 3b275b6..f0d83c6 100644
--- a/src/testdir/test_cmdline.vim
+++ b/src/testdir/test_cmdline.vim
@@ -378,6 +378,7 @@
   endfor
 
   call delete('Xtags')
+  set tags&
 
   call assert_fails('call getcompletion("", "burp")', 'E475:')
 endfunc