TST: correctly predict locale setability
Don't join the lang and encoding when performing the trial set, because

```
locale.setlocale(locale.LC_ALL, ('it_CH', 'utf-8'))
```

succeeds, but

```
locale.setlocale(locale.LC_ALL, 'it_CH.utf-8')
```

throws locale.Error.

Closes #5537.

Thanks! (and it's great that Travis builds PRs now) 
