g_date_time_format: improve support for alt digits

Improve a few situations where g_date_time_format() was getting the
padding wrong when displaying alt digits (eg: Arabic numerals) for
formatting time.

We now depend on nl_langinfo (_NL_CTYPE_OUTDIGITn_WC) to do the
conversion, which is very likely glibc-specific, but our previous method
relied on a glibc-specific printf() feature, so no harm done there.

Add a configure check for nl_langinfo (_NL_CTYPE_OUTDIGITn_WC).

Uncomment a few testcases that were failing previously.

https://bugzilla.gnome.org/show_bug.cgi?id=658107