patch 8.2.4149: test override not restored, autocommand left behind

Problem:    Test override not restored, autocommand left behind.
Solution:   Correct restoring test override.  Delete autocommand afterwards.

src/testdir/test_mapping.vim

@@ -1453,6 +1453,7 @@ func Test_map_script_cmd_survives_unmap()
 
   nunmap <F3>
   unlet b:result
+  autocmd! CmdlineEnter
 endfunc
 
 " Test for using <script> with a map to remap characters in rhs