Fix symbol names to be locale independent

We currently use upper() to present enum values, which are usually defined in
lower case in the typelib, in upper cases. However, upper() is locale
dependent, so that e. g. in tr_TR.UTF-8, "invalid" becomes "iNVALiD"
because Turkish has some extra variants of "i".

Use a local ASCII-only translate() call instead to avoid this. Thanks to Nils
Philippsen for the idea!

This also adds a test locale "te_ST@nouppera" which defines toupper('a') == 'a'.
Run the Enum tests under this locale to reproduce the bug and verify the fix.

https://bugzilla.gnome.org/show_bug.cgi?id=649165