tests: fix test-chg to ignore a warning about being unable to set locale
This is apparently coming from bash when bash is providing the sh that we're
using to execute the .sh file generated by run-tests for this test.
Bash on my machine:
```
$ sh --version
GNU bash, version 5.1.4(1)-release (x86_64-pc-linux-gnu)
Copyright (C) 2020 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software; you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.
$ sh
sh-5.1$ LC_CTYPE=unsupported_value echo hi
sh: warning: setlocale: LC_CTYPE: cannot change locale (unsupported_value): No such file or directory
hi
```
Differential Revision: https://phab.mercurial-scm.org/D10468
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