Use locale 'C' for running tests. This fixes #17423

Setting LC_ALL to C helps keep gdb's output consistent ('print' gives us expected output). This fixes #17423. I do not have access to a windows/mac machines to test this. I've only tested it on an x86_64 linux box.

Wow, nice find!

Could you add a doc-block explaining why we're doing this as well? You may also be able to collapse this to one line:

```make
export LC_ALL = C
```